Upstream has a small performance improvement around not iterating UserList, as well as the new experimental parallel scheduler. We should pull these into our vendored SCons, and provide a mechanism to light up the experimental parallel scheduler in our builds.

Upstream patches:

• https://github.com/SCons/scons/commit/59eb68b1503c9d84976918dd558ff05f41ee30e6
• https://github.com/SCons/scons/commit/5bb9066c5a711e32f150365c52c503b90368e8be