Get the current
ByteOffset in the input byte sequence of the
Decoder.
The
Decoder does not provide any facility to get at the input
data directly (since that is tricky with an incremental decoder). The
next best is this primitive which can be used to keep track of the
offset within the input bytes that makes up the encoded form of a
term.
By keeping track of the byte offsets before and after decoding a
subterm (a pattern captured by
decodeWithByteSpan) and if the
overall input data is retained then this is enables later retrieving
the span of bytes for the subterm.