Return a Temporary from *Binding::Wrap.

Returning a JS<T> is GC-unsafe.

This commit also includes some cleanup around Node and Document reflection.
Remove some dead code and fix some pyflakes warnings in CodegenRust.py.
Merge pull request #2801 from Ms2ger/wrap-temp

Return a Temporary from *Binding::Wrap.
Merge pull request #2807 from Ms2ger/pyflakes-2

Remove some dead code and fix some pyflakes warnings in CodegenRust.py.
Store proxy handlers in a static array rather than a hashtable per window.