Add Embedder API for Accessing Imports/Exports of a Module

[florianhartung]

This PR implements the missing two Embedder API functions module_imports and module_exports.

These functions are called on a ValidationInfo, and return that module's imports and exports, respectively.

This change also makes our linker fully modular, without it requiring direct field access to ValidationInfo linke before.Thus, it should be possible to move the linker to a separate crate now. What do we think about this?

Checks

• Using Nix
  • Ran nix fmt
  • Ran nix flake check '.?submodules=1'
• Using Rust tooling
  • Ran cargo fmt
  • Ran cargo test
  • Ran cargo check
  • Ran cargo build
  • Ran cargo doc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/execution/linker.rs	100.00% <100.00%> (ø)
src/lib.rs	100.00% <ø> (ø)
src/validation/mod.rs	95.39% <100.00%> (+0.26%)	⬆️

... and 1 file with indirect coverage changes

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