MAINT: interpolate: move function definitions from __fitpack.h to _fitpackmodule.c

closes gh-1767

The history is sort of preserved: github shows me as having written these C-to-Fortran wrappers (sigh), but `git blame -M -C -C` seems to detect the ground truth.

My secret agenda is to avoid a wall of warnings  about functions defined but not used I get in gh-3174 when trying to reuse a single routine from `__fitpack.h`.

In fact, I'm not sure what is the recommended layout of these C extension modules, would appreciate an advise.