Add option to compile salva2d with wasm-bindgen

[willcrichton]

Right now, salva2d only compiles with stdweb. Future momentum seems to be moving towards wasm-bindgen, so adding support like e.g. nphysics has seems natural.

I followed the same strategy used in nphysics (see their Cargo.toml). By default, the salva crate uses stdweb to preserve compatibility. A user of salva can switch to wasm-bindgen like so:

salva2d = { version = "whatever", default-features = false, features = ["use-wasm-bindgen"] }

If y'all are ok with this strategy, I can copy it over to salva3d as well.