Disallow int de-/encoding

[liamsi]

Amino allows to encode int for convenience and while the underlying varint encoding doesn't waste bytes on the wire it remains unclear how this translates to the existing proto3 types (int32, int64). Disallowing int would resolve this and probably is not a bad idea anyways (as int is platform dependant). The SDK and the tendermint team seem onboard with doing the necessary changes.

Alternatively, always assume (proto) int64 when int is used on the go side (and err on 32 b it platforms).