Use once_cell to cache stdlib compilation in wasm

[swernli]

This adds simple caching of the stdlib compilation during check calls using OnceCell. Since the stdlib is static in the wasm layer and the store does not need to be mutable (the compilation unit for the checked user's code is not added to it), this can be done once for the lifetime of the loaded package and reused over and over. In local testing, this produced a 95% perf improvement on the check calls performed as the user types (from 2.3 ms to 0.1 ms).

[swernli]

Of note for those keeping track of what dependencies we are pulling in: the once_cell crate has been under review for inclusion in the Rust stdlib, and was partially stabilized as of rust-lang/rust#105587 so is expected to be in the language as of Rust 1.70. This change intentionally uses only the portion of the API that was stabilized such that the dependency can be dropped and the stdlib implementation used instead once that version of Rust releases.