index_set.[ch] -- Index set utitlities
json_get.c -- parse_glist() can return an index set
parse_glist.[ch] -- parse the actual index set.
Makefile -- reflect the above changes
jg_gram.y -- TOK_INDEX is no TOK_INDEXSET.  But TOK_INDEXSET is really TOK_ATSIGN TOK_IDENT("indexset").  Resolve this someday