"FLOGF!" is supposed to treat its first argument as a format
string (but doesn't because that part isn't implemented currently).
That means running something like
```rust
FLOGF!(term_support, "curses var", var_name, "=", value);
```
That would rightly just print "curses var", ignoring the other
arguments.
By contrast, FLOG! is the literal "just join these as a string"
version.
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.
