Update from_buffer_copy to accept an IBufferProtocol

[slozier]

Part of #1404

[BCSharp]

The changes are fine except for WriteSpan used now in several places. The use is OK but the implementation of method itself caught my attention. It is much less efficient than what spans allow. Perhaps it can be rewritten to something like:

internal unsafe void WriteSpan(int offset, ReadOnlySpan<byte> value) {
    Span<byte> dest = new Span<byte>((void*)_data, _size).Slice(offset);
    value.CopyTo(dest);
    GC.KeepAlive(this);
}

[BCSharp]

Looks good to me!