Contract versioning

[sanity]

The contract API will evolve over time, and we should support a contract versioning mechanism to allow these changes in a backward compatible way.

Here is a suggested approach for representing the versions.

enum ContractContainer {
  Wasm(WasmAPIVersion),
  // Non-Wasm contracts supported here
}

enum WasmAPIVersion {
  V0_0_1 { wasm_code : Wasm, parameters : [u8] },
  V0_0_2 { wasm_code : Wasm, parameters : [u8] },
}

For serializing these enums I suggest using a variable length integer library such as varuint, for maximum flexibility.