It's not super fast to clone a function, but it is faster than
re-parsing the test case file it came from.
Some tests want to mutate the function, and there may be other tests in
the same script that need the original function.
Require this trait on the value type stored in an EntityMap to 'unlock' the
methods intended for primary entity maps that are allowed to create references
with the 'push method.
This prevents accidentally depending on these methods in secondary maps.
Create a secondary entity map with space reserved for a known range of entity
references.
Add dfg.num_ebbs() and dfg.num_insts() methods to provide capacities.
