This should speedup calls to functions, by allowing us to specialize calls to functions with free variables without penalizing calls to other functions.

• Calls to functions without freevars might be a tiny bit quicker as we don't need to check for freevars when initializing the locals.
• Calls to functions with freevars should be faster, as they can now be specialized.

However, there is a bit of yak-shaving involved:

1. It is necessary to keep a reference to the function in the frame. This is actually cheaper than before as it we now only need borrowed references to the builtins and globals, saving a few loads and stores.
2. In order to do 1, we need to create a new function for eval and friends, rather than just passing a FrameDescriptor.

Although the speedup is probably not significant this is worthwhile as it makes more operations explicit in the bytecode which what we want when optimizing.

https://bugs.python.org/issue44525