make closure caching work correctly, and make it performant for synthetic commands inside other commands

[eeeeaaii]

• when we cache closures in a command, we need to lookup bindings
• we don't want to just try to look up the binding and catch an exception if the binding doesn't exist, because that's expensive
• setting command text might seem like something that doesn't have to be efficient, until you realize many builtins synthetically generate command objects
• the method that checks for the existence of bindings doesn't take into account dereferencing
• to make it take into account dereferencing would require doing some convoluted stuff in the code

[eeeeaaii]

closing this and writing up two other bugs

[eeeeaaii]

erm never mind I think I can rephrase the TODO here:

closure cashing isn't correct because of performance, make it correct and make synthetic generation of commands inside other commands bypass caching error handling or something since we know we aren't passing in an invalid command