[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/FlexBuffers/Utils/Value.swift

@@ -0,0 +1,137 @@
+/*
+ * 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
+
+public struct Value: Equatable {
+
+  @usableFromInline
+  enum Union: Equatable {
+    case i(Int64)
+    case u(UInt64)
+    case f(Double)
+  }
+
+  var sloc: Union
+  let type: FlexBufferType
+  let bitWidth: BitWidth
+
+  @inline(__always)
+  private init() {
+    sloc = .i(0)
+    type = .null
+    bitWidth = .w8
+  }
+
+  @inline(__always)
+  init(bool: Bool) {
+    sloc = .u(bool ? 1 : 0)
+    type = .bool
+    bitWidth = .w8
+  }
+
+  @inline(__always)
+  init(v: UInt64, type: FlexBufferType, bitWidth: BitWidth) {
+    sloc = .u(v)
+    self.type = type
+    self.bitWidth = bitWidth
+  }
+
+  @inline(__always)
+  init(v: Int64, type: FlexBufferType, bitWidth: BitWidth) {
+    sloc = .i(v)
+    self.type = type
+    self.bitWidth = bitWidth
+  }
+
+  @inline(__always)
+  init(v: Double, type: FlexBufferType, bitWidth: BitWidth) {
+    sloc = .f(v)
+    self.type = type
+    self.bitWidth = bitWidth
+  }
+
+  @inline(__always)
+  init(sloc: Union, type: FlexBufferType, bitWidth: BitWidth) {
+    self.sloc = sloc
+    self.type = type
+    self.bitWidth = bitWidth
+  }
+
+  @usableFromInline
+  var i: Int64 {
+    switch sloc {
+    case .i(let v): v
+    default: 0
+    }
+  }
+
+  @usableFromInline
+  var u: UInt64 {
+    switch sloc {
+    case .u(let v): v
+    default: 0
+    }
+  }
+
+  @usableFromInline
+  var f: Double {
+    switch sloc {
+    case .f(let v): v
+    default: 0
+    }
+  }
+
+  static let `nil` = Value()
+}
+
+extension Value {
+  @usableFromInline
+  @inline(__always)
+  func elementWidth(size: Int, index: UInt64) -> BitWidth {
+    if isInline(type) {
+      return bitWidth
+    } else {
+      for byteWidth in stride(from: 1, to: MemoryLayout<UInt64>.size, by: 2) {
+        let _offsetLoc: UInt64 = numericCast(numericCast(size) &+ padding(
+          bufSize: numericCast(size),
+          elementSize: numericCast(byteWidth)))
+        let offsetLoc = _offsetLoc &+ (index &* numericCast(byteWidth))
+        let offset = offsetLoc &- u
+
+        let bitWidth = widthU(offset)
+        if (UInt32.one << bitWidth.rawValue) == byteWidth {
+          return bitWidth
+        }
+      }
+      return .w64
+    }
+  }
+
+  @inline(__always)
+  func storedPackedType(width: BitWidth = .w8) -> UInt8 {
+    packedType(bitWidth: storedWidth(width: width), type: type)
+  }
+
+  @inline(__always)
+  private func storedWidth(width: BitWidth) -> BitWidth {
+    if isInline(type) {
+      return max(bitWidth, width)
+    } else {
+      return bitWidth
+    }
+  }
+}