Make Remove with two arguments faster for internal lists

[fingolfin]

Before:

gap> l := List([1..100000], i->[i]);;
gap> while Length(l)>0 do Remove(l,Length(l)); od; time;
53937

After:

gap> l := List([1..100000], i->[i]);;
gap> while Length(l)>0 do Remove(l,Length(l)); od; time;
17

Resolves #767

[ChrisJefferson]

The method used to reject immutable lists and now accepts them. This mostly just means the error occurs in a different place, but it does make some changes, for example p := Immutable([1,2,3]); Remove(p,100); now works, when it used to break.

This doesn't greatly make a difference, but probably best to explicitly reject immutable lists to TryNextMethod, just to avoid changing behaviour unnessarily?

[fingolfin]

Thanks for the feedback! I've changed it to explicitly test for IsMutable. No point in bogging this patch down with changes to which methods are applied -- if we want to change that, it should be in a separate PR.

[frankluebeck]

Yes, InstallEarlyMethod looks like a good solution for this kind of performance problem.
The one argument version of Remove is already a kernel operation and again quite a bit faster than this early method.