Make iterator zero-copy

[nitely]

There are a couple of ways this can be implemented:

• Yield a slice and let the user do text[slice] or iterate over the slice or whatever
• Yield a View[T] (not yet part of stdlib)
• Keep yielding strings but also provide a graphemeLenAt, so zero cost abstractions can be created on top of it

graphemeLenAt would be really nice to have regarless. Done, but it's not faster than the iterator.

[nitely]

I'll have to wait until OpenArray can be used as a return value. But it seems this won't happen any time soon. In the meantime there is https://github.com/nitely/nim-strunicode