make the interface to looplets more uniform

[willow-ahrens]

SwitchVisitor and other callable visitors that are overloaded by looplets should become named functions, and named looplet functions should be given uniform names. also, clean up stepper code introduced in #309

[willow-ahrens]

lets' rename unfulred to instantiated, and insert it around every call to instantiate, rather than at the top of calls to instantiate.