[Swift] Memory usage fix (#8643)

Allows a complete reset for the underlying memory of the
_InternalByteBuffers within FlatBuffers and FlexBuffers.

tests/swift/Tests/Flexbuffers/FlexBuffersReaderTests.swift

@@ -15,9 +15,10 @@
  */
 
 import Common
-import FlexBuffers
 import XCTest
 
+@testable import FlexBuffers
+
 final class FlexBuffersReaderTests: XCTestCase {
 
   func testReadingProperBuffer() throws {
@@ -30,6 +31,29 @@ final class FlexBuffersReaderTests: XCTestCase {
     try validate(buffer: buf)
   }
 
+  func testReset() throws {
+    var fbx = FlexBuffersWriter(
+      initialSize: 8,
+      flags: .shareKeysAndStrings)
+    write(fbx: &fbx)
+
+    try validate(buffer: ByteBuffer(data: fbx.data))
+    XCTAssertEqual(fbx.capacity, 512)
+    fbx.reset()
+    XCTAssertEqual(fbx.writerIndex, 0)
+    XCTAssertEqual(fbx.capacity, 8)
+
+    write(fbx: &fbx)
+    try validate(buffer: ByteBuffer(data: fbx.data))
+    fbx.reset(keepingCapacity: true)
+    XCTAssertEqual(fbx.writerIndex, 0)
+    XCTAssertEqual(fbx.capacity, 512)
+
+    write(fbx: &fbx)
+    try validate(buffer: ByteBuffer(data: fbx.data))
+    XCTAssertEqual(fbx.capacity, 512)
+  }
+
   private func validate(buffer buf: ByteBuffer) throws {
     let reference = try getRoot(buffer: buf)!
     XCTAssertEqual(reference.type, .map)

tests/swift/Tests/Flexbuffers/Mocks.swift

@@ -32,7 +32,11 @@ func createProperBuffer() -> FlexBuffersWriter {
   var fbx = FlexBuffersWriter(
     initialSize: 8,
     flags: .shareKeysAndStrings)
+  write(fbx: &fbx)
+  return fbx
+}
 
+func write(fbx: inout FlexBuffersWriter) {
   fbx.map { map in
     map.vector(key: "vec") { v in
       v.add(int64: -100)
@@ -57,5 +61,4 @@ func createProperBuffer() -> FlexBuffersWriter {
   }
 
   fbx.finish()
-  return fbx
 }