Add missing EncodeAddress

[Ben Guidarelli]

Problem

At the moment we provide DecodeAddress and you can get EncodeAddress behavior by taking a slice and copying it into the type of Address (32 byte array) then calling .String() on it but it is less than obvious.

Solution

Implement EncodeAddress that does this for you and matches APIs of other SDKs

[Patrick Bennett]

Right, otherwise you have to play games like this:

var addr types.Address
copy(addr[:], pkVal[:])

which will not be obvious to many users and should be a 'conversion constructor' equivalent as part of types.Address methods.