Undo tuna_assert_static

This seems to trip useless warnings on my compiler, so
I'm junking it.
Hide time_t from view for #8 and #2.

Uses an opaque buffer instead of exposing time_t.
Uses memcpy and some runtime asserts to provide portable behavior.
Greatly reduces our public API dependencies on system headers
which should give a stable ABI and permit Fortran wrapping.

Closes #8.