Support unparsing lengthKind="prefixed" with lengthUnits="characters"

DAFFODIL-114 added support for lengthKind="prefixed", but does not yet support unparsing when lengthUnits="characters". However this is implemented can likely be shared with the implementation for dfdl:content-length and dfdl:value-length functions where the second paramter is "characters", which is also unimplemented.