This adopts the new function store, replacing the C++ version.
It also reimplements builtin_function in Rust, as these was too coupled to
the function store to handle in a separate commit.
Get some stuff out of the common module, which is growing large.
Also migrate the tests into "native" Rust tests so they will run in parallel.
We have to use an explicit setlocale() call to get a multibyte locale, for the
"crazy" tests.
