Extract UTF-8 truncated sequence detection to Bitstring class #714
Description
As BitString.isTruncatedUtf8Sequence(bytes, start)
Also rationalises findValidUtf8Length(bytes) helper function in :unicode.characters_to_list/1 to work in the same way as other helpers of the same name in the :unicode module, using the BitString.isTruncatedUtf8Sequence function in required locations to maintain functionality.