Add empty inline_invlist.c

Add empty inline_invlist.c

This will be used for things need to handle inversion lists in the three
files that currently use them.  I'm putting this in a separate hdr,
because inversion lists are very internal-only, so should not be grouped
in with things that there is an external API for.  It is a dot-c file so
that the functions can continue to be declared with embed.fnc, and
porting/args_assert.t will continue to work, as it looks only in .c
files.