Print warnings when allowlist doesn't match anything

[pvdrz]

This PR introduces a way to store warning messages inside Bindings so they can be emitted later. This allows bindgen to use cargo:warnining=MESSAGE in build scripts to show warnings and still produce warning log entries (which means this PR would fix #2167).

We still have some questions about this that we would like to resolve before promoting this to a non-draft PR:

• Should we emit log entries AND build scripts warnings at the same time? or should we favor one over the other?
  Apparently cargo won't print warnings for crates.io dependencies unless -vv is passed (Mechanism to unconditionally emit warnings from build scripts cargo#3777). I decided to float the warning messages all the way up and add a Bindings::take_warnings method so the user can print them directly in the build script. I also added a print_warnings macro to ease this process.

• Is there any functionality in the test suite to capture stdout/stderr to be able to test that the warnings are being printed correctly?

cc @amanjeev

Edit: As pointed out by @kulp (thanks!), the warning log entries are still emitted..

[kulp]

This seems reasonable to me, and if @emilio does not see a problem maybe we can merge it soon. You could rebase it down to a few commits that all pass the tests, or we could squash it when merging.

[pvdrz]

This seems reasonable to me, and if @emilio does not see a problem maybe we can merge it soon. You could rebase it down to a few commits that all pass the tests, or we could squash it when merging.

Thanks for your review :) I addressed your comments and rebased the PR so all commits pass the tests individually

[emilio]

Looks sensible, but I think we can do a couple simplifications.

[pvdrz]

@emilio I've addressed your comments. I'll squash them after your next review so you can still see the diff

[emilio]

Looks great, thanks!