WebIDL codegen: Replace cmake with a single Python script
When [playing around with Cargo’s new timing visualization](https://internals.rust-lang.org/t/exploring-crate-graph-build-times-with-cargo-build-ztimings/10975/21), I was surprised to see the `script` crate’s build script take 76 seconds. I did not expect WebIDL bindings generation to be *that* computationally intensive.
It turns out almost all of this time is overhead. The build script uses CMake to generate bindings for each WebIDL file in parallel, but that causes a lot of work to be repeated 366 times:
* Starting up a Python VM
* Importing (parts of) the Python standard library
* Importing ~16k lines of our Python code
* Recompiling the latter to bytecode, since we used `python -B` to disable writing `.pyc` files
* Deserializing with `cPickle` and recreating in memory the results of parsing all WebIDL files
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This commit remove the use of CMake and cPickle for the `script` crate. Instead, all WebIDL bindings generation is done sequentially in a single Python process. This takes 2 to 3 seconds.
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