disasm: Allow different static analysis backends?

[shazow]

There's a few disassemblers out there with varying constraints/features.

It could be a nice short-term approach to support third-party backends behind a unified whatsabi API. It would also help detect discrepancies across implementations, allow for comparative benchmarking, and help iteration/finding bugs.

Or if another backend is made to be good enough, whatsabi could cut some unnecessary code and replace maintenance burden with upstream collaboration. :)

Was recently pointed to https://github.com/acuarica/evm which seems quite nice (currently under heavy development), and possible a good compliment for a whatsabi backend.