[WIP] Auto-install Rust compiler toolchain for CI and JS people

Since we brought the WebAssembly MPD parser to fix performance issues we
had with some contents at canal+, we added another language (Rust) to
the project which is not compatible with most of the JS-world tools we and
other JS developers usually deal with.

Now that we added the `MULTI_THREAD` experimental feature and embeds,
the WebAssembly generation from that other language is even a required
step in our main build script (`npm run build`).

Consequently, when another developer or a CI needs to build an RxPlayer
(e.g. to test a development before a release is made), they have to
install:
  1. NodeJS + npm
  2. our JavaScript dependencies
  3. A Rust toolchain able to output WebAssembly. We generally go
     through `rustup` here basically an helper tool to install Rust
     toolchains
  4. `binaryen`, which is a collection of tools to transform that
     WebAssembly file.

(1) and (2) are very common use cases that both JS people and CI workflows
do very easily: they mostly already have nodejs + npm installed locally
(or it's very simple to set-up) and the second is only an `npm install`
call away.

(3) and (4) however, though not hard (you have to install rustup and
binaryen which is pretty simple on all platforms we relied on), are more
unusual dependencies that JS developers might be reticent to install and
that are more work to set-up on a CI.

Consequently I'm doing here a proof-of-concept where those tools are
auto-installed locally if not found when doing an `npm run build` call,
letting both people and CI produce a build easily without having to deal
with the dual-language complexity.

---

For rustup it's pretty straightforward as their install script already
take care of the platform detection, target installation and so on. All
I had to do is to make sure binaries were installed locally (`./tmp`) and
did not pollute the developers environment (e.g. no `PATH`
modification).

For binaryen however, it's less simple as they don't propose such
auto-install system.
however, I noticed that they propose prebuilt binaries for the most usual
platforms on their github's release page:
https://github.com/WebAssembly/binaryen/releases

The idea here (not yet finished), is to automatically detect the
platform of the user and fetch the right binaryen binary locally to then
rely on both it and the local rustup to produce builds in case the
command isn't available globally.

Also, because it will be faster and less error-prone for frequent
developers to install both of those globally once and for all, we print a
notice explaining what we do when that auto-install script is run.

scripts/build_wasm_release.sh

@@ -8,7 +8,7 @@ print_toolchain_installation_notice() {
   echo " |                                                                                  |"
   echo " |  If you intend to develop on the RxPlayer regularly, it is recommended that you  |"
   echo " |  install globally rustup (with the \"wasm32-unknown-unknown\" target) as well as   |"
-  echo " |  binaryen. Once done, please remove this \".tmp\" directory.                       |"
+  echo " |  binaryen. Once done, please remove this \"tmp\" directory.                        |"
   echo " +----------------------------------------------------------------------------------+"
 }