Add unresolve functions and declaration_to_names reverse index

[st0012]

Extracted from #589

Add methods for unwinding name resolution during incremental updates:

• unresolve_name: converts a Resolved name back to Unresolved
• unresolve_reference: detaches a reference from its declaration and unresolves the underlying name
• remove_name: removes a name and cleans up reverse indices

Add declaration_to_names reverse index that tracks which names resolve to each declaration. Populated during record_resolved_name, cleaned up by the unresolve/remove primitives.

[vinistock]

The changes look good, just trying to understand if we got the right data structure

[st0012]

Creating this method as we'll also update name_dependents here in follow up PRs.

[vinistock]

Yeah, let's start without any auxiliary maps and implement the simplest version.

We can iterate on performance once we have something that works and that we fully understand

[vinistock]

This is okay for now, but be careful with mixing the removal and the invalidation of the data.

The fact that a constant reference was removed doesn't necessarily mean that the NameRef needs to become unresolved. For example:

# file1.rb
class Foo
  # NameRef(CONST, parent_scope: None, nesting: Foo)
  CONST
end

# Delete
# file2.rb
class Foo
  # NameRef(CONST, parent_scope: None, nesting: Foo)
  CONST
end

[st0012]

My plan is for this and #589 to ALWAYS unresolve CONST when a Foo definition is removed to make sure this will be correct:

class Foo                                                                                                                                                               
  include Bar  # Bar defines CONST
  CONST        # resolves to Bar::CONST via ancestor lookup                                                                                                             
end           

And then we create a follow up PR to make this and other invalidation more efficient.