Implements verifier and code gen for swift (#6373)

Updates test cases on linux

Adhere to new protocol naming

Adds fuzzing

Adds documentation

Adds support for string unions

Updated fuzzer generated code

swift/Sources/FlatBuffers/FlatbuffersErrors.swift

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2021 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
+
+/// Collection of thrown from the Flatbuffer verifier
+public enum FlatbuffersErrors: Error, Equatable {
+
+  /// Thrown when buffer is bigger than the allowed 2GiB
+  case exceedsMaxSizeAllowed
+  /// Thrown when there is an missaligned pointer at position
+  /// of type
+  case missAlignedPointer(position: Int, type: String)
+  /// Thrown when trying to read a value that goes out of the
+  /// current buffer bounds
+  case outOfBounds(position: UInt, end: Int)
+  /// Thrown when the signed offset is out of the bounds of the
+  /// current buffer
+  case signedOffsetOutOfBounds(offset: Int, position: Int)
+  /// Thrown when a required field doesnt exist within the buffer
+  case requiredFieldDoesntExist(position: VOffset, name: String)
+  /// Thrown when a string is missing its NULL Terminator `\0`,
+  /// this can be disabled in the `VerifierOptions`
+  case missingNullTerminator(position: Int, str: String?)
+  /// Thrown when the verifier has reached the maximum tables allowed,
+  /// this can be disabled in the `VerifierOptions`
+  case maximumTables
+  /// Thrown when the verifier has reached the maximum depth allowed,
+  /// this can be disabled in the `VerifierOptions`
+  case maximumDepth
+  /// Thrown when the verifier is presented with an unknown union case
+  case unknownUnionCase
+  /// thrown when a value for a union is not found within the buffer
+  case valueNotFound(key: Int?, keyName: String, field: Int?, fieldName: String)
+  /// thrown when the size of the keys vector doesnt match fields vector
+  case unionVectorSize(
+    keyVectorSize: Int,
+    fieldVectorSize: Int,
+    unionKeyName: String,
+    fieldName: String)
+  case apparentSizeTooLarge
+
+  public static func == (lhs: FlatbuffersErrors, rhs: FlatbuffersErrors) -> Bool {
+    lhs.localizedDescription == rhs.localizedDescription
+  }
+}