inspect: gate behind experimental-inspect feature

[davidhewitt]

This is the last thing I want to do before preparing 0.18 release.

The pyo3::inspect functionality looks useful as a first step towards #2454. However, we don't actually make use of this anywhere within PyO3 yet (we could probably use it for better error messages). I think we also have open questions about the traits which I'd like to resolve before committing to these additional APIs. (For example, this PR adds IntoPy::type_output, which seems potentially misplaced to me, the type_output function probably wants to be on a non-generic trait e.g. ToPyObject or maybe #2316.)

As such, I propose putting these APIs behind an experimental-inspect feature gate for now, and invite users who find them useful to contribute a finished-off design.

[davidhewitt]

(Changelog skip just because I'll manually include this in the 0.18 release branch coming shortly, to simplify rebasing.)

[davidhewitt]

bors r+

[bors]

Build succeeded:

• conclusion