This is the approach used in easyproto
52d3ac4744/writer.go (L203)
It allows to occupy slightly less space for negative numbers.
The format is still protobuf, although, technically, this is a breaking
change for our stable marshaling format.
However, we don't use int32 at all and all enums have positive values,
so nothing is broken.
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>