Stellar: fix default_impl and ContractOverrides

[brozorec]

This PR:

• renames defaultimpl to default_impl in Non-Fungible
• adds ContractOverrides to imports in Enumerable

[CoveMB]

Sounds good, we can consider adding compilation tests so we can test the compilation of all different combination.

I started here we could replace running test with simply running cargo build and also catch warnings

[brozorec]

Sounds good, we can consider adding compilation tests so we can test the compilation of all different combination.

I started here we could replace running test with simply running cargo build and also catch warnings

That's great, it would've def caught those issues.