Improve memory accounting

[justjake]

• Enable malloc_usable_size and other previously linux-only features that Emscripten can actually emulate. This should improve memory accounting.
• Breaking: runtime.computeMemoryUsage() now directly returns a host Javascript object, instead of a handle. We no longer need a "system context" to pass the result back as a JSValue.
• remove runtime.getSystemContext() since it has no callers and is private
• Breaking: Use Emscripten STRICT=1 mode, dropping some properties previously exposed on the Emscripten module type
• Maybe breaking: Enable Emscripten WASM_BIGINT=1 mode, which may cause some internal functions to return bigint instead of number.

Fixes #206

[jsProj]

Just curious about any updates on this.

[justjake]

I haven’t made any progress on this. I usually find time to do maintenance on this project around Christmas, so that’s the earliest I would work on it.

I if I recall, I hit issues trying to convince Emscripten & quickjs to behave together the way I want — it should be possible but it seems to require more invasive patching of quickjs than I had patience for at the time.