Changed externref_table to use geometric resizing, giving amortized constant time additions

[richardlett]

Previously __wbindgen_externref_table_grow was called in alloc adding a constant size each time.

This results in taking Theta(N) time amortized to pass a single new JsValue to wasm, where N is number/size of existing items.

Changes

This pull request resizes geometrically, giving amortized constant additions to the externref_table.

Specifically, max(128, curr_len), so that we the larger of 128*usize, or the size of the current array.

This results in a growth factor of 2, the same as Rust's Vec growth factor

Testing:

It fixes #2291, where it took over 150 seconds to add 1.2 million JsValues's to a Vec on my PC with reference types enabled.
Now, it takes only 0.7-0.9 seconds.

---

Pros
Reduces complexity from Theta(N^2) to pass N JsValues to wasm to Theta(N)
Cons
Slightly more memory required -- After resizing, the table will be half full.

I'm setting this as draft, since this is my first PR! :)
All tests passed for me. Tell me if I missed anything!

[alexcrichton]

Thanks for the investigation here, and thanks for the PR! This does indeed look like a mistake on my end, 2x resizing is definitely intended here rather than "always add a fixed amount". This solution looks good to me, would you ok landing it or did you want to add any more tests?

[richardlett]

No problem! I think its all good - Submitted as draft, as I was unfamiliar with the pull request system. 👍

[alexcrichton]

Ok no worries, thanks again!