Add an abstract type for wallet.Sig

[manoranjith]

Location

[wallet]

Problem

Currently wallet.Sig is defined as a variable size byte array in the abstract wallet package. However, the length of the signature depends upon the signature scheme that we use, which in-turn depends upon the wallet backend we use. Hence, in order to Encode/Decode a wallet.Sig, we directly use the wallet.Sig.Encode / wallet.Backend.DecodeSig` functions.

The problem is that, these Encode/DecodeSig functions do two different things:

1. Convert the blockchain specific concrete data to/from binary format (byte array).
2. Write/Read the data in binary format to/from the wire.

Proposal

As we did for Address, Action, Data and other types (see #257), we can change the definition of wallet.Sig to binary.Unmarshaler and binary.Marshaler. Then

1. The wallet backend can implement the MarshalBinary, UnmarshalBinary functions. that convert these types to/from binary format.
2. These type can then be directly decoded using perunio.(En|De)code functions.

Another point to be noted

This will also, remove the usage of perunio package in the channel and wallet packages in backend/ethereum. Using perunio package in these places is a concern because, perunio will now be treated as a specific implementation of the wire protocol and these packages should not directly depend on a specific implementation of wire protocol. Also, since we have introduced (Un)marshalBinary type of each of the abstract types like Action, Address, Data, App (see #257) and Sig (this issue), these functions can be used to convert these types to/from their binary representation.

[manoranjith]

@matthiasgeihs

[matthiasgeihs]

Relates to / duplicate of #168