Directly construct KCFGs instead of parsing

[ehildenb]

Currently (as of #1372), we are generating KCFGs as the proof obligations for each foundry proof. We do this by generating a giant K file with the proof obligations, kompiling it, the using kprove --emit-json-spec ... --dry-run on the same file to read in the specs and generate KCFGs from them.

Instead, we can just directly generate the KCFGs right after calling kompiling the definition, and never bother writing these claims to the giant K file. This skips an entire round trip of unparsing + parsing the claims.

See #1372 (comment)