tlv/truncated: add truncated integer encodings
tlv/truncated: add truncated integer encodings

This commit adds the truncated integer encodings used in the
variable-size onion payloads. The amount and cltv delta both use the
truncated encoding to shave bytes in the overall size, and will likely
be used in the future for additional extensions where size is a
constraint.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd