Add methods to code_typed to handle Function and Method arguments.

I want to start porting stuff from TypeCheck.jl into Base. My plan is to do this in a series of small to medium PRs as I have time, so that I can get feedback on whether you want various functions in Base (without there being an overwhelming amount of code at a time). After the separately useful helper functions are in, I'll make a pull request to add a `julia -w` mode to lint a file(s).

If you'd rather I do this in a different way, let me know. :) 

The TypeCheck repo is here: https://github.com/astrieanna/TypeCheck.jl