stability lint: check stability of macro uses, check bodies at def site

[aturon]

The stability infrastructure should apply to macros in two ways:

• It should be possible to provide a stability level for a macro definition. This should trigger the lint on any use of the macro.
• The body of a macro should be linted at the definition site.

Of the two, the first is much more important. But it may require additional infrastructure in the AST to track e.g. the def_id of the macro that was expanded. (Currently, the expansion span information tracks only the name and source location of the macro.)

[alexcrichton]

cc #15703

[huonw]

cc #17316

[steveklabnik]

Triage: i am unaware of significant changes to any of this in a long time.

[Mark-Simulacrum]

Since, for example, select! is unstable, I think this is at least partially fixed. I think that the second item on the list isn't done, but I'm also not entirely sure what it means.