_This PR replaces #1364_

This PR introduces a feature called _hierarchical namespaces_, a
module-like system that maintains backward compatibility. It is the
foundational groundwork that will pave the way for:
1. Naked namespaces (files without namespace declarations)
2. Package publishing and sharing
3. Explicit item imports
4. Exports and re-exports
5. and the rest of the stuff
[here](#493 (comment))

Key changes include:
1. `VecIdent` type introduction: This abstraction over `Vec<Ident>`
represents the new namespaces in the AST. Alternate names could be
`DotIdent` or `Path`, but those names were already loaded with meaning
in our codebase. I'm open to feedback on nomenclature here.
7. `NamespaceId` usage: This replaces our previous flat map-style
`Rc<str> -> Res` item tracking.
8. `NamespaceTree`: This tracks the namespace hierarchy separately in
low-cost data structure, and allows us to keep our basic `tys` and
`terms` maps relatively untouched.
9. Name resolution algorithm update: The algorithm in `resolve.rs`
received a heavy facelift. Not only did I try to simplify the existing
code, but I also added the required functionality to check candidate
partially-opened or re-opened namespaces.

This PR maintains 100% backward compatibility with the existing
resolution engine, preserving features like re-opening namespaces,
shadowing semantics, merging namespaces via aliases, and absolute-path
opens. It also introduces the ability to reference a function in a
different namespace if its parent namespace is opened.

Additional notes:

1. Katas used a form of shadowing that wasn't tested in our compiler.
This PR [adds a
test](https://github.com/microsoft/qsharp/pull/1312/files#diff-7a3aa16f18f6d132b9e54a6b6ebd53e6c3b41cea805c8f8db9c456b533a3c96bR2787)
for that.
2. Pursuing #1321 could help differentiate user-facing and
compiler-engineer-facing string representations of structs, which are
increasingly conflated.
3. There's potential for refactoring `ast`, `fir`, and `hir` to use
shared generic types, reducing duplicate `Ident`/etc types.
4. Completions functionality remains the same, but it's not yet aware of
the ability to partially open namespaces (e.g. `open Microsoft.Quantum`.
This will be addressed in future updates.
5. There is a performance regression, but we are performing a full
tree-traversal namespace resolution algorithm in a place where we used
to just do a constant table lookup. I have optimized this as much as I
believe is reasonable for now, bringing the regression down from ~40%,
where it was originally, to around 5-10% based on the benchmark run.

Also closes #1336

---------

Co-authored-by: Mine Starks <mineyalc@microsoft.com>
Co-authored-by: Mine Starks <16928427+minestarks@users.noreply.github.com>