Make type checker save type information

[n-osborne]

This PR proposes to marshall the product of the type checker and save it in a file with the .gospel file extension so that external tool does not have to run the type checker by themselves.

[shym]

This is a simple and nice change, well done!
I’ve pushed in my drop-ok branch two extra commits to complete the change of dropping OKs and to make dune aware of the .gospel files that will be generated for succeeding tests. In turn, this makes an extra test that type checking succeeded on documentation :-)

I have one suggestion: it would be nice to teach gospel check how to process .gospel files, among other things as a way to test the read_gospel_file and that the marshalling did work as expected. It would also serve as a simple way to inspect the content of a .gospel file.

[n-osborne]

Thank you @shym for your comment :-)
I've integrated your two commits and, following your suggestion, made gospel check handle .gospel files and test that reading back the typing information from a .gospel file give the same output than typing from the initial .mli file.

[shym]

This is nice, thank you!
(I only reviewed the diff with my drop-ok branch, not commit by commit)
A tiny suggestion: with GNU’s wc, wc -l < bar is just the line count, so you don’t need an extra cut).
I’m not sure about portability (but that’s already the case with the cut, I might argue :-).

[mariojppereira]

I believe this pull request is ready to be merged.

[n-osborne]

Thanks.
Merging.