ensure LEB-encoded integers are in canonical form

[phritz]

https://docs.google.com/document/d/12xNPzVCPSC2bTv7myxNRoMGx79AqFHQb89LfmRcFpGc/edit#bookmark=id.hb76ig9956ej

I think this might just be as simple as canonicalizing the bits we get from big.Int in {attofil, bytesamount, channelid, etc}. Maybe they are already canonicalized, but doesn't hurt to add a check. I doubt (?) whether it's worth checking when decoding if the representation is canonical, seems like wasted work.

[phritz]

given filecoin-project/specs#131, this is likely no longer relevant as we might not be leb128 encoding integers at all. if that is true, we can close this