Can we make String into an Iter?

[richardpawson]

At the moment, n Iterwe can do a lot of things with aStringthat we can do to anIter` for example:

• each loop
• index & range

but we can't apply e.g. filter.

We can call asIter() on String and calling e.g. filter on the result compiles, but does not run.

First prize: make String work as an Iter and make it work with the Hofs. If we get this we should then get rid of the distinction between the 'contains' methods for String and Iter.

Runners up:

• Make asIter turn a String into a genuine iterable.
• Prevent calling asIter on a String with a compile error.

[richardpawson]

Superseded by #720