encode delete op as negative numbers

[dwelle]

Is there a reason why the lib doesn't encode delete ops as -10 instead of {d:10}? Saves three bytes if stored as string. If stored as binary, the benefits are even bigger I presume.

EDIT: just noticed that it was maybe added in anticipation of invert support (at the time I forked this lib to move to delete ops encoded as negative numbers, this was not the case).