[EXPERIMENT] Use SmallVec in GoalStalledOn

[nnethercote]

It makes GoalStalledOn bigger, reversing #160005, but that's not a problem any more because #160479 massively reduced the number of GoalStalledOn copy operations, and inline operations are cheaper.

[nnethercote]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 96a3828 (96a3828266908011b1cba8c784812b8986180686)
Base parent: 8fa1c96 (8fa1c96cfd489e4c27654c144ae871ce2c4db6c6)

[rust-timer]

Finished benchmarking commit (96a3828): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.5%	[0.3%, 0.9%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.7%	[-0.8%, -0.7%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 1.9%, secondary 2.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.9%, 2.0%]	2
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.9%	[1.9%, 2.0%]	2

Cycles

Results (secondary -6.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.2%	[-6.2%, -6.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 458.344s -> 465.594s (1.58%)
Artifact size: 398.95 MiB -> 398.90 MiB (-0.01%)

[nnethercote]

A tiny icounts regression, some possible cycles improvements.

@jdonszelmann, are you still planning that alternative ugly-but-effective enum representation for GoalStalledOn::{stalled_vars, sub_roots}?