Swift update performance benchmark infrastructure (#7255)

* Keep the underlying storage capacity when clearing the FlatBufferBuilder. Gives a significant performance boost for serialisation of many small messages.

* Use Googles Swift benchmark library for more consistent results and dynamic number of iterations, simplification of tests as result.

Co-authored-by: Joakim Hassila <hassila@users.noreply.github.com>

tests/FlatBuffers.Benchmarks.swift/Package.swift

@@ -24,9 +24,11 @@ let package = Package(
   ],
   dependencies: [
     .package(path: "../../swift"),
+    .package(url: "https://github.com/google/swift-benchmark", from: "0.1.0"),
   ],
   targets: [
     .target(
       name: "FlatBuffers.Benchmarks.swift",
-      dependencies: ["FlatBuffers"]),
+      dependencies: ["FlatBuffers",
+                     .product(name: "Benchmark", package: "swift-benchmark")]),
   ])