Convert Browser.workers to HandleAllocator. NFC

[sbc100]

Using the common/shared HandleAllocator for all subsystems allows us to make improvements in the central location. For example, we can play around using externref_t instead of allocating integers on the JS side.

This also comes with codesize saving, at least for test_codesize_hello_dylink_all (I imagine because it already includes several HandleAllocator usages).

This also removes a field from the global Browser object.

[kripken]

This avoids using a raw array and allows reusing IDs

How important are those goals? Workers are created quite rarely so I suspect that keeping this code minimal with a simple array is good enough. Did you see a problem somewhere?

[sbc100]

This avoids using a raw array and allows reusing IDs

How important are those goals? Workers are created quite rarely so I suspect that keeping this code minimal with a simple array is good enough. Did you see a problem somewhere?

Yes, that isn't really the goal here. The goal is more about using the HandleAllocator consistently so if/when we improve it (or perhaps just delete in favor of managing externref_t) on the native side, we have single point of control where we all subsystems can benefit.

[sbc100]

Updated the PR description.