1021: Wasm dependencies r=matklad a=detrumi
As a first step towards running RA on WASM (see #1007), this tweaks the dependencies somewhat so that projects built using `wasm-pack` can use `ra_ide_api` as a dependency.
There were two problems:
- use of undeclared type or module `MmapInner`
This error occurred because of the `memmap` crate, as a dependency of `fst`
Solution: specify `default-features = false` for the `fst` package (see https://github.com/BurntSushi/fst/issues/70)
- use of undeclared type or module `imp`
This happened in the `wait-timeout` crate ([which uses `Command` under the hood](https://github.com/alexcrichton/wait-timeout/issues/18)), a dependency of `rusty-fork` which is a dependency of `proptest`.
Solution: move `proptest` to dev-dependencies and add `#[cfg(test)]` to the `test_utils` crate.
**Edit:** Oh, that causes trouble with resolving the import when running the tests. Hmm...
Co-authored-by: Wilco Kusee <wilcokusee@gmail.com>
