Fix: resolution structs and import aliases to support separated namespaces

[LesterEvSe]

Currently, SimplicityHL allows separated namespaces. A user should be able to use the exact same name for both a type and a function in the same scope. For example:

type temp = u32;
fn temp() {}

The previous alias tracking functionality broke this behavior during the use import resolution phase.
So, This PR refactors the alias architecture and the process_use_item logic.

[KyrylR]

Was this thing allowed in simplicity 0.4.1?

[KyrylR]

Was this thing allowed in simplicity 0.4.1?

I was able to compile this:

fn foo() -> bool { false }
type foo = bool;

fn main() {
    let x: foo = false;
    assert!(foo());
}

[KyrylR]

Suggested change

	// NOTE: Consider to optimising this with `Vec` instead of `HashMap`
	// TODO: Consider to optimising this with `Vec` instead of `HashMap`

[KyrylR]

At the same time the following test passes:

    #[test]
    fn test_private_alias_error_does_not_mask_duplicate_function_import() {
        let (graph, _ids, _dir) = setup_graph(vec![
            ("libs/lib/A.simf", "pub fn foo() {}"),
            ("libs/lib/B.simf", "pub fn foo() {} type foo = u32;"),
            ("main.simf", "use lib::A::foo; use lib::B::foo;"),
        ]);

        let mut error_handler = ErrorCollector::new();
        let program_option = graph.linearize_and_build(&mut error_handler).unwrap();
        let errors = error_handler.to_string();

        assert!(
            program_option.is_none(),
            "build should fail when a second import reuses the function name `foo`"
        );
    }

[KyrylR]

ACK 763086b

So I would mention the behavior above in the md doc

[LesterEvSe]

Was this thing allowed in simplicity 0.4.1?

I was able to compile this:

fn foo() -> bool { false }
type foo = bool;

fn main() {
    let x: foo = false;
    assert!(foo());
}

Yes, SimplicityHL 0.4.1 allows this

[LesterEvSe]

At the same time the following test passes:

    #[test]
    fn test_private_alias_error_does_not_mask_duplicate_function_import() {
        let (graph, _ids, _dir) = setup_graph(vec![
            ("libs/lib/A.simf", "pub fn foo() {}"),
            ("libs/lib/B.simf", "pub fn foo() {} type foo = u32;"),
            ("main.simf", "use lib::A::foo; use lib::B::foo;"),
        ]);

        let mut error_handler = ErrorCollector::new();
        let program_option = graph.linearize_and_build(&mut error_handler).unwrap();
        let errors = error_handler.to_string();

        assert!(
            program_option.is_none(),
            "build should fail when a second import reuses the function name `foo`"
        );
    }

It is okay too. I added this test and explain why it passed

[KyrylR]

ACK feb901e