ast+topdown+planner: allow for mocking built-in functions via "with" (#4540)

With this change, we can replace calls to built-in functions via `with`. The replacement
can either be a value -- which will be used as the return value for every call to the
mocked built-in -- or a reference to a non-built-in function -- when the results need
to depend on the call's arguments.

Compiler, topdown, and planner have been adapted in this change. The included
docs changes describe the replacement options further.

Fixes first part of #4449. (Missing are non-built-in functions as mock targets.)

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>