Instantiation of literals#26

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("...")`.