* Remove byte* property in ByteBufferAllocator.
This allows consumers to read/write into native memory, but without
having to always pin the managed `byte[]` when working with managed
memory. This allows for users to not need to Dispose() ByteBuffers
when they are using the default ByteArrayAllocator class.
Instead, we use `Span<byte> GetSpan()` methods to get access to the
underlying memory buffer.
Fix#5181
* Add a set of benchmark tests.
* Add ReadOnly spans.
This allows consumers to use ReadOnlyMemory<byte> as the backing storage
for ByteBuffers, which is useful in read-only scenarios.
* Run tests using ENABLE_SPAN_T in appveyor.
* Fix FlatBuffers.Test.csproj to work on older MSBuild versions.
* Change the test script to test UNSAFE_BYTEBUFFER
* Address PR feedback.
Remove IDisposable from ByteBuffer.
* Respond to PR feedback.
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