[Swift] Flexbuffers native swift port (#8577)

* Offical Swift port for FlexBuffers

This is the offical port for FlexBuffers within
swift, and it introcudes a Common Module where code
is shared between flatbuffers and flexbuffers.

Writing most supported values like maps, vectors,
nil and scalars into a flexbuffer buffer. And includes
tests to verify that its similar to cpp

* Reading a flexbuffer

Implementing reading from a flexbuffer, enabling
most of the buffers features, like most types, maps, vectors,
typedvectors, and fixedtypedvectors.

Currently, if an offset/object cant be read we default to a swift
nil instead of the default flexbuffers 'null' with all values.

* Fixes bazel breaking due to new project structure

Address warnings within the library

* Adds comment on why we added the code & properly enforce the amout of bytes needed

swift/Sources/Common/Int+extension.swift

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2024 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+extension Int {
+
+  /// Moves the current int into the nearest power of two
+  ///
+  /// This is used since the UnsafeMutableRawPointer will face issues when writing/reading
+  /// if the buffer alignment exceeds that actual size of the buffer
+  @inline(__always)
+  public var convertToPowerofTwo: Int {
+    guard self > 0 else { return 1 }
+    var n = UInt32(self)
+
+    #if arch(arm) || arch(i386)
+    let max = UInt32(Int.max)
+    #else
+    let max = UInt32.max
+    #endif
+
+    n -= 1
+    n |= n >> 1
+    n |= n >> 2
+    n |= n >> 4
+    n |= n >> 8
+    n |= n >> 16
+    if n != max {
+      n += 1
+    }
+
+    return Int(n)
+  }
+}

swift/Sources/Common/Scalar.swift

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+/*
+ * Copyright 2024 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+/// A boolean to see if the system is littleEndian
+public let isLitteEndian: Bool = {
+  let number: UInt32 = 0x12345678
+  return number == number.littleEndian
+}()
+
+/// Constant for the file id length
+public let FileIdLength = 4
+
+/// Protocol that All Scalars should conform to
+///
+/// Scalar is used to conform all the numbers that can be represented in a FlatBuffer. It's used to write/read from the buffer.
+public protocol Scalar: Equatable {
+  associatedtype NumericValue
+  var convertedEndian: NumericValue { get }
+}
+
+extension Scalar where Self: FixedWidthInteger {
+  /// Converts the value from BigEndian to LittleEndian
+  ///
+  /// Converts values to little endian on machines that work with BigEndian, however this is NOT TESTED yet.
+  public var convertedEndian: NumericValue {
+    self as! Self.NumericValue
+  }
+}
+
+extension Double: Scalar {
+  public typealias NumericValue = UInt64
+
+  public var convertedEndian: UInt64 {
+    bitPattern.littleEndian
+  }
+}
+
+extension Float32: Scalar {
+  public typealias NumericValue = UInt32
+
+  public var convertedEndian: UInt32 {
+    bitPattern.littleEndian
+  }
+}
+
+extension Bool: Scalar {
+  public var convertedEndian: UInt8 {
+    self == true ? 1 : 0
+  }
+
+  public typealias NumericValue = UInt8
+}
+
+extension Int: Scalar {
+  public typealias NumericValue = Int
+}
+
+extension Int8: Scalar {
+  public typealias NumericValue = Int8
+}
+
+extension Int16: Scalar {
+  public typealias NumericValue = Int16
+}
+
+extension Int32: Scalar {
+  public typealias NumericValue = Int32
+}
+
+extension Int64: Scalar {
+  public typealias NumericValue = Int64
+}
+
+extension UInt: Scalar {
+  public typealias NumericValue = UInt
+}
+
+extension UInt8: Scalar {
+  public typealias NumericValue = UInt8
+}
+
+extension UInt16: Scalar {
+  public typealias NumericValue = UInt16
+}
+
+extension UInt32: Scalar {
+  public typealias NumericValue = UInt32
+}
+
+extension UInt64: Scalar {
+  public typealias NumericValue = UInt64
+}

swift/Sources/Common/padding.swift

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+/*
+ * Copyright 2024 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+/// Gets the padding for the current element
+/// - Parameters:
+///   - bufSize: Current size of the buffer + the offset of the object to be written
+///   - elementSize: Element size
+@inline(__always)
+public func padding(
+  bufSize: UInt,
+  elementSize: UInt) -> UInt
+{
+  ((~bufSize) &+ 1) & (elementSize &- 1)
+}