Add deprecation infrastructure

[Robert P. Goldman]

Develop infrastructure to support deprecating features we are removing from ASDF.

Would require one or more conditions.

We expect that the first step would be to raise a STYLE-WARNING, then a real WARNING (which could cause a build to fail), then an ERROR.

Probably it would be helpful to also supply some macros that would make deprecation easier (deprecated alternatives for defining forms?).

[Robert P. Goldman (rpgoldman)]

Fare suggests making a deprecation library part of UIOP, and simply using it in ASDF.

[Faré (fahree)]

See my obsolete-function-warnings branch.

[Faré (fahree)]

This is being worked on in MR !38 https://gitlab.common-lisp.net/asdf/asdf/merge_requests/38