Reduce next-solver memory usage by interning CanonicalQueryInput

[laundmo]

Goal: Reduce memory usage of next-solver especially when exceeding recursion limit

Changes:

• Added Interner::CanonicalInput associated type and surrounding interning machinery (mostly copied from ExternalConstraints)
• Changed Cx::Input to be I:CanonicalInput and adjusted other places in the code to match, including calling the interning during canonicalize_goal

Results:
According to Heaptrack (and btop) peak memory usage for compiling bevy_render (like #161748) with next-solver enabled was reduced from ~15GiB to ~4GiB since most of it was the HashMap index for NestedGoals (and search_graph::GlobalCache), which used to be 56 bytes (IIRC), but now is a single reference thanks to Interned.

Closes (maybe): #161748
Tracking issue: rust-lang/goals#113 and #160895 (comment)

r? @lcnr

[rustbot]

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[ShoyuVanilla]

@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: 9d1834c (9d1834c69e38df691a50a3052833ce4cb83252c2)
Base parent: 5321a4f (5321a4f40c957cf3587c055e77461febc2ebc865)

[rust-timer]

Queued 9d1834c with parent 5321a4f, future comparison URL.
There are currently 0 preceding artifacts in the queue.
It will probably take at least ~1.0 hours until the benchmark run finishes.