Implement serialization protection via magic

[JustusAdam]

What Changed?

This adds a u64 magic number to the beginning of the output artifact from paralegal-flow which is verified upon loading the artifact.

The magic number is a hash of the modification dates of all files in the paralegal-spdg crate.

Why Does It Need To?

At present there is no protection that the format expected by a policy application conforms to the format written by the flow analyzer. Specifically the analyzer may be compiled against an older or newer version of paralegal-spdg. In such cases the derived serde implementation for the output artifact may differ. bincode, the serialization library we use, does not offer protection against this case, leading to out-of-memory errors and memory leaks in deserialization.

The magic hash added detects this version mismatch and reports it.

Checklist

• 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.