Add erf and erfc

See merge request !318

They don't need to be visible outside of the single compilation unit,
which will help the C compiler a bit.

If the codegen has produced a Collection with (say) some static
arrays, we can't assume that the callee program ast has an fdecl
property. So if it's a collection, spin over the contents.

iname_to_tag -> iname_to_tags

Pylint

See merge request inducer/loopy!313