`now` exposed when neither `wasm-bindgen` or `stdweb` are available

[dodomorandi]

When neither wasm-bindgen or stdweb features are used, the now function exposes a foreign now function, explicitly making it safe.

This behavior produces the effect that the FFI now is exposed even if wasm::now is never called, just because it is extern "C". Unfortunately, this causes some unexpected issues in other crates. I propose to remove wasm::now if no valid features are available, even if I understand that this is a breaking change.

Another possible solution is to change the behavior of the features in order to activate wasm-bindgen by default if the target is wasm32-unknown-unknown and stdweb is not specified.

I am very open to discussion, even because the rationale behind blindly using an extern now function is not clear to me. If you want I will be happy to open a PR when I have a clearer idea of what's the best thing that could be done.

[repi]

We've run into this as well, from the usage of this crate by parking_lot. That higher level issue is also tracked in Amanieu/parking_lot#269