feat(api) Merge js-api into api

[Hywan]

Description

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.

How to review?

I think browsing https://github.com/wasmerio/wasmer/tree/js-api-into-api/lib/api/src can provide a nice overview.

Then the biggest contribution of this patch is the new Cargo.toml, https://github.com/wasmerio/wasmer/blob/js-api-into-api/lib/api/Cargo.toml. The trick is

wasmer/lib/api/Cargo.toml

Lines 43 to 47 in 9a5a10f

	#####
	#
	# # Dependencies and Development Dependencies for `sys`.
	#
	[target.'cfg(not(target_arch = "wasm32"))'.dependencies]

to avoid conflicts with wasm-bindgen. Dependencies are grouped in 3 categories: Shared, ones for sys, ones for js.

The second biggest contribution is the lib.rs, https://github.com/wasmerio/wasmer/blob/js-api-into-api/lib/api/src/lib.rs. The trick to support wasm-bindgen correctly, is to declare the crate-type programmatically in lib.rs rather than in Cargo.toml as such:

wasmer/lib/api/src/lib.rs

Lines 29 to 30 in 9a5a10f

	#![cfg_attr(feature = "js", crate_type = "cdylib")]
	#![cfg_attr(feature = "js", crate_type = "rlib")]

Then, there is compiler errors that are raised if some features aren't correctly used, like:

wasmer/lib/api/src/lib.rs

Lines 458 to 461 in 9a5a10f

	#[cfg(all(feature = "js", not(target_arch = "wasm32")))]
	compile_error!(
	"The `js` feature must be enabled only for the `wasm32` target (either `wasm32-unknown-unknown` or `wasm32-wasi`)."
	);

The rest is only moving files.

Next steps

As soon as this PR is merged, and probably as soon as #2460 is merged, then we could imagine refactoring lib/api/ to remove duplicated code. Now it is possible :-).

[Hywan]

bors try

[bors]

try

Build failed:

• Test on windows-x64

[Hywan]

bors try

[bors]

try

Build failed:

• Test on linux-x64

[Hywan]

bors try

[bors]

try

Build succeeded:

• Audit
• Code lint
• Test on linux-x64
• Test on windows-x64
• Test on macos-x64
• Test cross-compile on linux
• Test cross-compile on macos