Fix detection of GAP strings

[fingolfin]

... by using IS_STRING_REP: this then also covers sorted and unsorted strings. Of the following six strings, the old code only accepted the first two.

gap> t:="str";; TNUM_OBJ(t);
72
gap> t:=Immutable("str");; TNUM_OBJ(t);
73
gap> t:="cba";;IsSet(t);; TNUM_OBJ(t);
74
gap> t:=Immutable("cba");;IsSet(t);; TNUM_OBJ(t);
75
gap> t:="abc";;IsSet(t);; TNUM_OBJ(t);
76
gap> t:=Immutable("abc");;IsSet(t);; TNUM_OBJ(t);
77

(My secret true motivation for this is removing the last use of IMMUTABLE in a GAP kernel extension, to prepare the ground for gap-system/gap#6025.)