NSAttributedString/NSString constructor from StringBuilder

[Therzok]

Steps to Reproduce

1. Use a StringBuilder to construct a string
2. new NSAttributedString(sb.ToString())
3. Feel powerless when allocations can't be avoided

Expected Behavior

There should be a way to construct the NSAttributedString/NSString from the StringBuilder chunks, instead of having to map the entire string in memory.

Actual Behavior

The opposite.