Derivation of trait implementations

[yannbolliger]

In theory, #[derive(SomeTrait)] works already now, because the macro simply generates an impl SomeTrait for X block, which we can deal with quite well.
However, the most frequently used traits like Clone, PartialEq generate code that uses very advanced features which stainless doesn't yet support.

In summary: we support the implementation derivation but not the resulting implementations.

=> Discuss: Should we try to support some of the most useful traits, especially Clone?