Make contract a versioned enum

```rust
/// A versioned wrapper for contracts
/// It proxies to the inner type
enum Contract<'a> {
V0(ContractV0<'a>)
}
```
It should implement encode + decode

where `ContractV0` is the current version
https://github.com/cennznet/pact/blob/05b2a57eba9e79ddad4da2db7f494da7701b07b5/src/compiler/mod.rs#L47

Start in src/compiler/mod.rs around line 47 and read the existing ContractV0 definition and its surrounding compiler code. Check the project's existing encoding and decoding conventions, then define the versioned Contract wrapper around the current version. Done means the wrapper supports both operations while preserving the V0 contract representation.

Written by the indexing model from the issue text.