Some functions in blister.c ignore mutability

[wilfwilson]

(Resolving this issue should wait until #3385 and #3391 are merged, so that the whole file can be done at once by adding the appropriate RequireMutable calls.)

As pointed out initially by @fingolfin in #3385 (comment):

...most other functions in [blister.c] also lack [mutability checks] :-/, e.g. FuncUNITE_BLIST. So I'd also be fine with accepting this "bug" in this PR for now, and then follow it up soon with another PR which adds the missing RequireMutable calls to all blister.c functions, together with test cases. And then we put that as a "wrong result" bug fix into the release notes, or so.

Here one of the instances of this bug in live action:

gap> l1:=MakeImmutable(ListWithIdenticalEntries(10,true));
[ true, true, true, true, true, true, true, true, true, true ]
gap> l2:=MakeImmutable(ListWithIdenticalEntries(10,false));
[ false, false, false, false, false, false, false, false, false, false ]
gap> UniteBlist(l2, l1);
gap> l2;
[ true, true, true, true, true, true, true, true, true, true ]