Change constant.py to constant.json

[sequencer]

The background is in the RocketChip, we need metadatas for instruction decoding, rather than instruction name and bitpat only. For downstreams like us, which need to parse instruction metadata from riscv-opcodes. I think it might be good to change constant.py to parse the constant.json(or some structural text file) to make other projects being able to directly use these constant to generate metadatas.

[Shehrozkashif]

interested to solve this issue assign it to me please