Instantiation of literals

[dtolnay]

Before introducing a thread_local WipFunction in #13, the only way for us to attach literals into the right WipFunction was by constructing them through methods of MakeFunction, as in f.unit() and f.string("...").

Now that we have a thread_local way to track what goes where, these literal constructors would make more sense as plain functions rather than methods of MakeFunction: reflect::unit() and reflect::string("...").