feat(traits): allow multiple traits to share the same associated function name and to be implemented for the same type

[nickysn]

Description

This change allows a type to implement two different traits that have the same associated function name. Additionally, this change allows to have the same function name in a "impl struct", as well as in a trait.

Problem*

1. Previously, it was not allowed for two traits to share a common function name and to have implementations for the same type. For example, the following code would produce a "duplicated function" error:

    trait Trait1 {
        fn tralala();
    }
    trait Trait2 {
        fn tralala();
    }
    struct MyStruct { }
    impl Trait1 for MyStruct {
        fn tralala() {
        }
    }
    impl Trait2 for MyStruct {
        fn tralala() {
        }
    }

After this change, the code compiles. It is not yet possible to call <MyStruct as Trait1>::tralala() or <MyStruct as Trait2>::tralala(), but this will done in a future PR. This PR is a preparation for that. Also, calling MyStruct::tralala() should produce an error, stating that the compiler doesn't know which function to call. The proper error will also be implemented in a future PR. For now, in this case the compiler says that the function doesn't exist.
2) Previously, it was not allowed for a struct to have a function in an "impl struct" block and a function in a "impl trait for struct" with the same name. For example, the following code would also produce a "duplicated function" error:

    struct MyStruct { }
    impl MyStruct {
        fn tralala() {
        }
    }

    trait MyTrait {
        fn tralala();
    }
    impl MyTrait for MyStruct {
        fn tralala() {
        }
    }

In this case, calling MyStruct::tralala() will call the function from "impl MyStruct", not the function from the trait.

Summary*

In order to support multiple functions (and, in the future, types) with the same name, coming from different traits, we had to change the way ItemScope stores names. The structure was changed from HashMap<Ident, (ModuleDefId, Visibility)> to HashMap<Ident, HashMap<Option<TraitId>, (ModuleDefId, Visibility)>>. This adds another dimension to the symbol tables - instead of just a name, it now becames name + optional trait id.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

👍