Find good inlining heuristics

We have the ability to inline, and a simple inlining pass using it. That pass inlines only methods called just once, to avoid code size issues. Even that simple rule ends up not having clear benefits in either code size or speed - examples go both ways, so the pass is off by default.

We should investigate this and find good heuristics for inlining. The opportunity here is potentially large, as we can process large compilation units efficiently (and emcc runs us on the whole codebase currently, because of that) so this would mean our inlining is LTO-like in being able to span C compilation units.

The issue names no files, tests, or entry points; start by locating the existing inlining pass and its current single-call rule. Compare candidate heuristics against both code size and speed, using the issue's examples and compilation-unit behavior as context. Done means identifying a heuristic with clear benefits and evidence that the pass should be enabled.