Make wasm-bindgen optional for wasm32 target

[Wonshtrum]

Trying to embed biscuit-auth compiled to wasm32 is a pain outside of JavaScript due to wasmbindgen.
It generates a lot of boilerplate, which is tedious and error-prone to mock correctly.
In this PR I propose to make the old wasm feature optional for the wasm32-unknown-unknown target and rename it wasm-bindgen to avoid confusion.
The only necessary import is for time, which is implemented through performance_now with wasmbindgen. I propose, for a pure wasm32 target, to create a simple instant_now symbol returning a u64, meant to represent a monotonic timestamp, and let the user provide the right implementation in their environment.

note: we could keep the name wasm for the feature making it a non breaking change, but I find it pretty confusing.