Companion

Answer a mutable copy of object that also only holds 21-bit characters.

Create an object of the appropriate size, whose descriptor is an instance of TwentyOneBitStringDescriptor. This can store any Unicode character, including those from the Supplemental Planes. Run the generator for each position in ascending order to produce the code points with which to populate the string.

Create a new mutable twenty-one-bit string with the specified number of elements.