toke.c: Allow \N{} handling fcn to be used elsewhere in core

toke.c: Allow \N{} handling fcn to be used elsewhere in core

This function will be used in regcomp.c in a later commit.  This commit
changes the function so that it is callable outside of toke.c.  It adds
a parameter and moves some code in new_constant to the wrapper function
so that these do not cause problems when called from outside toke.  And
it adds some assertions