Move poly1305_get_tag() into poly1305.c, hide poly1305_init().

Move poly1305_get_tag() into poly1305.c, hide poly1305_init().

The crypto library on Windows exposes a symbol named poly1305_init(),
which clashes with ours. We can avoid this by moving poly1305_get_tag()
to poly1305.[ch], where it belongs better, and this allows us to make
all the lower-level Poly1305 functions static.

Also remove the support for associated data while we are at it, since we
are never using it.