Extract most code from define_feedable!

[Zalathar]

This PR extracts most of the non-trivial code from the define_feedable! macro (which defines the TyCtxtFeed::$query methods), and moves it to a helper function query_feed_inner written in ordinary non-macro code.

Doing so should make that code easier to read and modify, because it now gets proper IDE support and has explicit trait bounds.

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There should be no change in compiler behaviour.

I've structured the commits so that the actual extraction part is mostly just whitespace changes, making it easier to review individually with whitespace changes hidden.

[rustbot]

r? @jackh726

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[Zalathar]

No perf changes intended, but let's check:

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

☀️ Try build successful (CI)
Build commit: b135ecc (b135ecc5b5ca9773965d5c7368568e9fe2f0f49c, parent: 1a3cdd34629306fa67624eaa60d73687e7fcf855)

[rust-timer]

Queued b135ecc with parent 1a3cdd3, future comparison URL.
There are currently 3 preceding artifacts in the queue.
It will probably take at least ~3.6 hours until the benchmark run finishes.