Refactor repo into Cargo workspace with scaffolding for Rust-based queries

[willcrichton]

What Changed?

The repository has become a Cargo workspace. The new crates directory contains:

• dfpp: PDG generation, same as before
• dfpp-explorer: the dfpp/explorer directory factored into a standalone crate
• dfgraph: the core ProgramDescription type emitted by dfpp
• dfcheck: the runtime for property checkers

I also added a props directory at the root which contains sample implementations of properties for Websubmit and Lemmy.

Why Does It Need To?

To separate out the different concerns, and to ensure that third-party property checkers don't need to include every dfpp transitive dependency.

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
  • The new dfcheck and dfcheck-cli crates are not documented because their structure is highly tentative.
• Documentation in Notion has been updated
  • Not sure where to update with this information?
• Tests for new behaviors are provided
  • Need to add some tests that run the dfcheck-cli tool on the sample properties.
• Refactor rustc serializers to just wholesale convert from rustc types to ProgramDescription before any serialization happens

[JustusAdam]

Nice work with the refactoring. Here are some thoughts I had on this code so far.

[JustusAdam]

I'm dot sure how much I agree with the df prefix. Since we're introducing new names we should make it consistent with the paper. I would suggest we call the graph crate spdg, and the checker policy-check. dfpp we can leave for now since people are already used to that.

[JustusAdam]

I'm dot sure how much I agree with the df prefix. Since we're introducing new names we should make it consistent with the paper. I would suggest we call the graph crate spdg, and the checker policy-check. dfpp we can leave for now since people are already used to that.

Uhhh, maybe we could rename the dfpp create to spdgen? What do you think? cargo spdgen?

[JustusAdam]

What about the build.rs for the properties? Are those actually necessary? And weren't you going to copy the build.rs to the explorer also?