Instantiation of literals

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("...")`.

Start by tracing the existing MakeFunction methods f.unit() and f.string("...") and the thread_local WipFunction introduced in #13. Update the literal-construction API to use reflect::unit() and reflect::string(...), then verify that literals still attach to the correct WipFunction.

Written by the indexing model from the issue text.