Renamed miterator to old_miterator (as it is obsolete).
A library of generic iterators.

The code comes mainly from tests/fpiterator-focused.mz, cleaned up and
with a new [concat] function.
New implementation of iterators on lists.

These iterators can be wrapped to generic iterators (of type [iterator::iterator]).
Remove the internal [max] function in mutableTreeMap

[max] is defined in corelib/int.mz
Merge branch 'master' into jonathan
Fix [fpiterator-focused.mz], remove [tests/iterator.mz].
Added a [fold_stop] function in stdlib/iterator.mz

[fold_stop] is a fold function with the extra possibility for the user
to stop the iteration at any time. The user function [f] returns a
couple of the new accumulator and a boolean that indicates if the
iteration shall continue or not.
Rename [fold_left] to [fold] in stdlib/iterator.mz

We don't have the choice of the direction: we cannot write a
[fold_right] function or something, so i thing it's pointless to
indicate the direction of the iteration in the name.
Add a comment about a dead branch.
Merge branch 'master' of git+ssh://yquem/~protzenk/public_html/mezzo
Merge branch 'master' of git+ssh://yquem/~protzenk/public_html/mezzo
Cleanup.

[focused] is defined using [osf::wand] now
Merge branch 'master' of git+ssh://yquem/~protzenk/public_html/mezzo
Fix stdlib/iterator.mz

Note the more explicit manipulations in [map], because we have to give
an witness. I don't think this is bad - with a pack we see more
clearly how it works.
Cleanup (remove unnecessary parthesis)
In iterator, kept just the interruptible versions of iter and fold,
so as to avoid exporting too many variants. Defined fold in terms
of iter.
Changed the types of [none] and [some] in corelib/option, so that
explicit type applications are not needed as often.
Used none/some instead of None/Some in a couple places.
Found a trick in {iteration,iterator}::filter which replaces three
lines of annotations with just one line. If I understand correctly,
the type-checker is unable to auto-pack when the correct witness is
p * q, but it can auto-pack when the witness is just p or just q, so
we supply a hint that forces it to auto-pack in two steps.
Merge branch 'master' of git+ssh://yquem/~protzenk/public_html/mezzo