This has quite a lot of fall out. But the majority of it is just type or
UDL substitution, where the changes just fall through to other function
calls.
By changing property key storage to UTF-16, the main affected areas are:
* NativeFunction names must now be UTF-16
* Bytecode identifiers must now be UTF-16
* Module/binding names must now be UTF-16
~2% of the Speedometer 2.1 profile was just repeatedly performing the
shape transitions to add these two properties. We can avoid all that
work by caching a premade shape.
