Reintroduce zigzag encoding via tag

[liamsi]

As per #238 ints, int32s, and int64s aren't zigzag encoded by default. Add a tag such that developers can still zigzag encode. If they need to encode ints which often contain negative numbers this is more efficient.

TODOs:

• add a tag
• add back tests (see: Release 0.14.0 #240 (comment))

Zigzag encoding is used in proto3 for encoding sint(32|64).
References:

• https://developers.google.com/protocol-buffers/docs/encoding?csw=1#types
• https://developers.google.com/protocol-buffers/docs/proto3#scalar

[liamsi]

removed from the proto milestone: My understanding is, we require everything amino encodes can be represented as a protobuf message and not that amino supports every proto feature.