Proposal: make time varint encoded (instead of fixed length)

[liamsi]

In protobuf there is https://github.com/golang/protobuf/blob/14aad3d5ea4c323bcd7a2137e735da24a76e814c/ptypes/timestamp/timestamp.pb.go#L99-L108
where seconds and nanos are encoded as varint. See also the corresponding proto file / message:
https://github.com/golang/protobuf/blob/14aad3d5ea4c323bcd7a2137e735da24a76e814c/ptypes/timestamp/timestamp.proto#L121-L133

In amino, we currently encode time as fixed length (signed) ints:

go-amino/encoder.go

Lines 123 to 136 in 2106ca6

	err = encodeFieldNumberAndTyp3(w, 1, Typ3_8Byte)
	if err != nil {
	return
	}
	err = EncodeInt64(w, s)
	if err != nil {
	return
	}
	err = encodeFieldNumberAndTyp3(w, 2, Typ3_4Byte)
	if err != nil {
	return
	}
	err = EncodeInt32(w, ns)

I think it could be worthwhile to change this behaviour. Then, people working on amino in other languages, or interefacing via vanilla protobuf could just use proto's encoding / types.

[liamsi]

Closed via #224