narrowspec: move module into core
Having support for parsing the narrow specification in core is
necessary for moving many other parts of narrow to core.
We do still want to harmonize the narrow spec with the sparse
spec. And the format needs to be documented. But this shouldn't
hold up the code moving to core.
Differential Revision: https://phab.mercurial-scm.org/D2201
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors