Remove non-const Sequence::begin()/end() in internal code

Remove non-const Sequence::begin()/end() in internal code

... to avoid hidden cost of multiple COW checks, because they
call getArray() internally.
This obsoletes [loplugin:sequenceloop].

Also rename toNonConstRange to asNonConstRange, to reflect that
the result is a view of the sequence, not an independent object.

TODO: also drop non-const operator[], but introduce operator[]
in SequenceRange.

Change-Id: Idd5fd7a3400fe65274d2a6343025e2ef8911635d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/123518
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>