rhg: in path_encode, split Dest into VecDest and MeasureDest
Two separate types make the write semantics easier to understand
because we can consider the two sinks separately.
Having two independent compiled functions for size measurement
and for actual encoding seems likely to improve performance, too.
(and maybe we should get rid of measurement altogether)
Getting rid of [Dest] also removes the ugly option rewrapping
code, which is good.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.