generate
Produce an Avail string from the given generator of Unicode code points. Attempt to use a ByteStringDescriptor representation first, but generalize it to a TwoByteStringDescriptor or even a TwentyOneBitStringDescriptor as needed.
Parameters
size
The number of code points that will be in the resulting string, which is also the number of times the generator will be called.
generator
A function that takes a one-based index and produces a code point to write to the string at that index. The indices will be passed in ascending order, from 1 to size.