Support WASM std targets.

[gkgoat1]

This PR adds (currently untested) WASM std target support, which rely on an external linker, specified in WASM.md.

[gkgoat1]

Note that, currently, WASM support from this PR also has a much higher MSRV.

Update: I fixed the MSRV.

[dtolnay]

Thanks for the PR!

This is different enough from what the rest of the crate does that I prefer if you would publish this in a different crate for now. I am interested to see if others find this usable.

[gkgoat1]

Thanks for the PR!

This is different enough from what the rest of the crate does that I prefer if you would publish this in a different crate for now. I am interested to see if others find this usable.

Okay, published. I can wait until there becomes a need before submitting a followup PR.