Add proper WASM support feature flags and tests

[dingxiangfei2009]

This PR will supersede #32 if preferred.

This PR proposes the following changes:

1. Enable proper WASM flags in the dependencies
2. Remove parking_lot feature flag from once_cell, as suggested by its author matklad. In summary, recent once_cell has adapted to better work with std mutexes. See the discussion here
3. Adapt the tests to run with wasm-pack by conditionally using wasm-bindgen-test

[wigy-opensource-developer]

I loved the declarative macro for switching between different test proc_macros. Was there a reason you did not use them for a few files?

[maciejhirsz]

Cheers! Would be great to expand CI to also run the tests on Wasm, but that can come later.

[maciejhirsz]

Published in 0.8.2.