Contract self impl

Allow self impl for contracts as a shorthand for an anonymous ABI spec.
The point is to not have to write an ABI type for very small contracts.

This:

```sway
contract;
impl Contract {
    fn foo() -> u64 { 0 }
}
```

Should produce the same binary and ABI spec as:

```sway
contract;
abi MyAbi {
    fn foo() -> u64;
}
impl MyAbi for Contract {
    fn foo() -> u64 { 0 }
}
```