* Faster Write implementation
- Use Unsafe.CopyBlockUnaligned to copy bytes to the
WritableBuffer. This is temporary until we get newer
corefx bits with a better span.CopyTo implementation.
- Remove WritableBufferExtensions from Performance project
- Split method into WriteFast and WriteMultiBuffer
- Cache the span for the common case where
the buffer is non empty.
- Use ref locals instead of pinning pointers in fast path
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