adding all the methods required the BytesRef class and tests. Adding ICharSequence even though it is possible to use IEnumerable<char> instead.
adding CharsRefBuilder and tests for CharsRef.  Ensuring all current *.cs files have license in header.  Adding custom enumerator for CharsRef and BytesRef.
cleaning documentation for ICharSequence, BytesRef, CharsRef, CharsRefBuilder
adding SetOnce<T> with tests.