Seperate out AXI and XML generator tools

[rachitnigam]

The -b xilinx and -b xilinx-xml options to the Calyx compiler are not real backends---they act more like tools that ingest an input Calyx program and generate an AXI controller and XML information based on it. We should move them into different cargo projects that are solely focused on generating this stuff instead of pretending that they are backend.

[sampsyo]

Good call; making this a separate binary would be nice. One could fantasize that this greater separation could be a first step toward implementing more of the AXI stuff in Calyx instead of Verilog…