Auto-complete should be type-sensitive

We can do a better job of showing only the auto-complete options that might fit in a certain context. For example, we could look around the current cursor position to see if the user is typing a function call, and if so, look up the type of the function, and only show function names if they might eventually lead to the desired result type.

This would necessarily be a heuristic, since the user is typing the program, so it's guaranteed to not be syntactically correct most of the time; but there's a lot we can do.