feat(Cabal, cabal-install): Display -O and -g as numeric levels instead of booleans

[zlonast]

-- TODO: [code cleanup] The following is a hack. The "optimization" and
-- "debug-info" fields are OptArg, and viewAsFieldDescr fails on that.
-- Instead of a hand-written parser and printer, we should handle this case
-- properly in the library.

-- TODO: The following is a temporary fix. The "optimization"
-- and "debug-info" fields are OptArg, and viewAsFieldDescr
-- fails on that. Instead of a hand-written hackaged parser
-- and printer, we should handle this case properly in the
-- library.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.

[Mikolaj]

@ffaf1 notices this seems to change the behaviour of cabal (namely, it's output). If so, a different PR template should be used, which asks for a different set of tasks.

Would it be possible to add a test that prints the -O and -g things? Preferably in an early commit, so that it illustrates the modification, showing how it requires a change in the test result.