This patch takes the entire `wasmer-js` crate and merges it into the
`wasmer` crate.
Inside the `lib/api/src/` directory, there are 2 new directories:
1. a new `sys` directory, which contains the usual `wasmer` crate
implementation,
2. a new directory `js`, which contains the implementation of
`wasmer-js`.
The `Cargo.toml` file is still compatible. The `default` feature
fallbacks to `sys-default`, which enables the `sys` feature. All
features related to compilers or engines or anything else prior this
patch, activates the `sys` feature.
Parallel to that, there is a `js-default` and `js` features.
The `Cargo.toml` file is extensively documented to explain what are
dependencies, dev-dependencies, features and other sections related to
`sys` or to `js`.
There is a bug with `wasm_bindgen_test` where it doesn't compile or
look for tests in `tests/*/<test>.rs`. The hack is to name files
`tests/js_<test>.rs`. Ugly, but it works.
