Implement Extend/FromIterator for ArrayString

[clarfonthey]

Fixes #119. Caveat: takes the most conservative approach and doesn't extract any characters when there are less than 4 bytes left in the array.

[bluss]

Hey, thanks for working on this. The pain point I've seen here before is exactly how do deal with capacity and error handling, since the two traits are not designed to do that. I think that this solution of leaving up to 3 unused capacity in a byte array, while understandable, is too confusing to use in arrayvec, and would in fact often go counter to the user's wish(?), say if they want to fit 10 ascii letters into 10-capacity array.

[clarfonthey]

So to clarify, should I just close this PR then? Or would it be good to leave open as an option for the future?

[bluss]

Yeah, let's close this for now. I think we haven't found good rules for how it should work.