Add a `--check` flag

[srdtrk]

I don't like github workflows that generate code and push, which go-codegen can be used to create. Instead, I like workflows that check validity.

Therefore, I want to add a --check flag where go-codegen checks whether or not the output file provided (with -o flag) matches the output of the program.

[srdtrk]

Unfortunately, it seems the code generation is not deterministic, so a naive implementation doesn't work.