Add support for nested prefix lengths

DAFFODIL-114 added support for prefixed lengths, but did not implemented nested prefix lengths. Parsing should work without any modifications (one the nested SDE is removed) but unparsing still requires updates. The difficulty is that nested prefix lengths results in nested suspensions, which is not supported in the current unparse design. All suspensions split off of the main state. We either need to support nested suspensions, or figure out a way to make it so the suspensions aren'tnested.