Remove byte* property in ByteBufferAllocator (#5191)

* 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.