Add Wasm support

[daxpedda]

This adds support for the wasm32-unknown-unknown target.

When cfg(target_feature = "atomics") is not active, it will always return 0, which is appropriate, considering that without it you can't spawn threads.

Support for wasm32-unknown-emscripten could be added via pthreads, but I'm not qualified to do that.
WASI support cannot be added at the moment, as the wasm32-wasi-threads target doesn't exist yet.

Fixes #10.

[daxpedda]

Restarting CI, spurious network errors ...

[daxpedda]

Still just spurious network errors on CI.

In any case, the PR is fine, it shouldn't affect the CI at all.

[ruuda]

Ah yes, CI … last time I needed to release a new version of this crate, Travis CI still existed 😅 I’ll try to migrate this to GitHub Actions.

[daxpedda]

Thanks for restarting, it almost worked this time, only one spurious network error 😆!

I’ll try to migrate this to GitHub Actions.

Let me know if you need any help!

[ruuda]

Thanks for restarting, it almost worked this time, only one spurious network error

I had to restart it four or five times to get all of them to pass, I wonder what is causing it. But anyway, this change looks good, thanks a lot! I’ll get CI sorted out and then publish a new release.