Add imports functionality

[KyrylR]

Motivation (copied from #258)

The main goal of this PR is to enable an analogue of the OpenZeppelin library to exist in the SimplicityHL ecosystem. By introducing official compiler support for imports, we significantly increase code reusability and speed up application development. For projects heavily focused on security, this also allows developers to eventually "flatten" the code and audit all functions without external dependencies.

Checklist

• Visibility: Added pub and pub use to control local scope versus re-exporting.
• Import Syntax: Implemented parsing and resolution for use m::name; and explicit aliasing (as).
• Collision Handling: Enforced strict errors for local scope collisions, utilizing linearization for resolving re-export conflicts.
• Transitive Dependencies: Add support for libraries importing other libraries.
• Documentation: Сode comments are updated accordingly.
• Quality Assurance: Code is formatted, all tests pass, and CI shows no errors.

Related PRs (in order of the merge): #260, #256, #265, #266, #269, #278, #279, #280, #283, #287, #284, #289

[KyrylR]

Reopened instead of #258

[apoelstra]

026c0ff needs rebase

[LesterEvSe]

Note for reviewers: This PR does not follow a strictly additive structure. It is composed of several smaller PRs, each of which has a linear change history on its own. However, when combined, some things introduced in an earlier sub-PR are intentionally removed or superseded in a later one as the design evolves.

For example, #265 adds PathBuf to several variants in the enum Error, but #280 removes them due to the current error handler architecture.

[KyrylR]

ACK 8b12ac6; code review and test runs (see #260, #256, #265, #266, #269, #278, #279, #280, #283, #287, #284, #289)

[delta1]

ACK 8b12ac6; tested locally