Fix a bug where calling NextIterator on a "done" list iterator caused it to seemingly switch back to "not done" status

[wilfwilson]

This resolves #4226, reported by @james-d-mitchell. See the discussion there for further context.

Currently, iterators behave differently for immutable dense lists as opposed to other lists w.r.t. whether NextIterator throws an informative error when called on a finished iterator. Moreover, in one of the cases, calling NextIterator on a finished iterator unnecessarily broke the value of IsDoneIterator.

I tried making the error-checking behaviour consistent, but this led to an unacceptable 10% slowdown. Instead I've just removed the breaking of IsDoneIterator, and found one or two ways to speed up the code (mostly by reducing access to record components).

If speed of iterators is important, then I think these changes are useful. If speed is not important, then perhaps I should reinstate proper error checking in NextIterator_DenseList...

I've added some tests, removed some commented-out code, and rearranged a bit of code too - I'm happy to remove either of the latter two for being off-topic.

Quick experiments:

# Mutable list
gap> for i in [1..10] do iter := Iterator([1 .. 10000000]);; for i in iter do od; od; time;
20926 # Before this PR
17429 # After this PR

# Immutable list
gap> for i in [1..10] do iter := Iterator(Immutable([1 .. 10000000]));; for i in iter do od; od; time;
13205 # Before this PR
13158 # After this PR

[fingolfin]

Thank you, this PR is exemplary -- nicely separated clean commits make it a breeze to review. 5/5, would review again ;-)