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` file
* Deserializing with `cPickle` and recreating in memory the results
  of parsing all WebIDL files

----

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.

components/script/dom/bindings/codegen/CodegenRust.py

@@ -54,32 +54,6 @@ RUST_KEYWORDS = {"abstract", "alignof", "as", "become", "box", "break", "const",
                  "use", "virtual", "where", "while", "yield"}
 
 
-def replaceFileIfChanged(filename, newContents):
-    """
-    Read a copy of the old file, so that we don't touch it if it hasn't changed.
-    Returns True if the file was updated, false otherwise.
-    """
-    # XXXjdm This doesn't play well with make right now.
-    #       Force the file to always be updated, or else changing CodegenRust.py
-    #       will cause many autogenerated bindings to be regenerated perpetually
-    #       until the result is actually different.
-
-    # oldFileContents = ""
-    # try:
-    #     with open(filename, 'rb') as oldFile:
-    #         oldFileContents = ''.join(oldFile.readlines())
-    # except:
-    #     pass
-
-    # if newContents == oldFileContents:
-    #     return False
-
-    with open(filename, 'wb') as f:
-        f.write(newContents)
-
-    return True
-
-
 def toStringBool(arg):
     return str(not not arg).lower()