Introduce the symbol collection context and a symbol collection step.

[tritao]

Description

This PR introduces a new symbol collection step to the compiler pipeline, that is done after parsing but before type checking.

It collects identifiers for the top level entities in the AST and allows referencing through a parsed declaration id.

These will be used in a subsequent PR that introduces a new AST order resolving pass that runs before type checking, thus allowing us to get rid of the deferred monormophization "hack", simplifying some things and eliminating some bugs.

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.

[tritao]

Rebased to latest master.

[IGI-111]

LGTM, I appreciate that this is well commented