It is common to represent a function as a String, and previously that
required re-validating the UTF-8 in a Vec<u8>. The fmt::Write trait
writes UTF-8 directly into a String, so no extra checking is required.
This also means we can implement Display for Function which gives it a
to_string() method. This makes the function_to_string() method
redundant, so delete it.
The functions in the write module are no longer generally useful, so
make the module private. The Display trait on Function is all we need.
It's not super fast to clone a function, but it is faster than
re-parsing the test case file it came from.
Some tests want to mutate the function, and there may be other tests in
the same script that need the original function.
