Fletcher v0.0.17

This release improves the Fletcher bus infrastructure for writing.

• Adds a write response channel.

This allows to ensure bytes have landed in memory when the response "ok" bit is asserted.
Once ArrayWriters signal on the unlock stream that a command is completed, the bytes are now guaranteed to be as written in memory.

• Adds a write request last bit.

This bit signifies a BufferWriter has sent the last write request for a command, allowing platform-specific implementations to perform some final action.
One example is the fletcher_opae platform that requires a write fence to be produced.
This is somewhat analogous to how ArrayBuilders in Arrow's various libraries have a Finish() function.
On the AXI top level, this bit is exposed through the awuser(0) bit.