Module with multiple #[odra::module] impl block does not compile

[kpob]

By now the only correct implementation of a module is having a struct and a single implementation block. If for some reason, a developer would like to have a module with more than one implementation block marked as odra::module, compilation will fail.
We need to come up with a solution that gently informs a user that he should put all the entry points in a single impl block or allow having multiple blocks.

[kpob]

I'll go first with a possible solution. The most straightforward approach is to wrap up the whole module with mod, then the macro can process struct and all impl blocks at once.

[kpob]

#87 may resolve the issue