Add a new name field on DeclId and remove some unnecessary lookups in the DeclEngine

[emilyaherbert]

Description

This PR is a subset of #3744. It adds a name field to DeclId which contains the name of the declaration. This is helpful in that it allows us to bypass lookups in the DeclEngine for trivial name info and is also a subset of #3744.

This PR also removes/refactors some of these unnecessary lookups in the DeclEngine in light of the new name field.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[mohammadfawaz]

Awesome.