Fixes found when experimenting with Lemmy

[JustusAdam]

What Changed?

This PR contains a series of fixes for bugs in new features found when using the new Paralegal version on an unhacked Lemmy.

• Support fn's in sub-closure stubs
• Adds strategic overtaint for stubs to avoid crashes and to recognize that the return value isn't used exactly as returned from the closure. The type of overtaint is a loss of field level precision.
• External markers, the build config as well as the include and analyze argument now contribute to the hash used to control recompilation
• Added handling for InstanceDef::FnOnceShim
• Renames flow-models to stubs
• Applies stubs in marker discovery
• Fixes ordering of shortest_path
• Ensure we check marker reachability in non-crate-local functions too.

Why Does It Need To?

Fixes bugs.

Makes the names consistent. What we called "flow models" here are more like stubs.

Checklist

• Fix shortest_path ordering
• Above description has been filled out so that upon quash merge we have a
  good record of what changed.
• New functions, methods, types are documented. Old documentation is updated
  if necessary
• Documentation in Notion has been updated
• Tests for new behaviors are provided
  • New test suites (if any) ave been added to the CI tests (in
    .github/workflows/rust.yml) either as compiler test or integration test.
    Or justification for their omission from CI has been provided in this PR
    description.