Proptest set up with CI.

[YoshikiTakashima]

Description of changes:

This PR sets up the stubbed proptest fork with CI. No stubs have been merged yet.

Resolved issues:

Resolves #1363, #1608

Call-outs:

• The number of files diffed are large b/c of (1) license and (2) rustfmt

Testing:

• How is this change tested? All tests except 2 are included in this PR.

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[YoshikiTakashima]

@danielsn Moving the discussion here: Cargo audit finds several license differences as well as 1 potential vuln. Not sure what to do since I am not familiar OSS license issues.

error[L001]: failed to satisfy license requirements
[544](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:545)
  ┌─ atty 0.2.14 (registry+https://github.com/rust-lang/crates.io-index):4:12
[545](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:546)
  │
[546](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:547)
4 │ license = "MIT"
[547](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:548)
  │            ^^^
[548](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:549)
  │            │
[549](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:550)
  │            license expression retrieved via Cargo.toml `license`
[550](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:551)
  │            rejected: not explicitly allowed

error[L001]: failed to satisfy license requirements
[2983](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:2984)
  ┌─ fuchsia-cprng 0.1.1 (registry+https://github.com/rust-lang/crates.io-index):5:15
[2984](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:2985)
  │
[2985](https://github.com/model-checking/kani/runs/7814066681?check_suite_focus=true#step:4:2986)
5 │ files-expr = "BSD-3-Clause"

[YoshikiTakashima]

@danielsn After some thought, I think proptest itself could be the first canary project.

proptest contains a large number of test cases that are implemented with the proptest! macro. As of now, none of them are run using kani. However, once the GH actions get merged, we can run kani in this library's CI. There will be no need to override the import b/c proptest macro is already part of the codebase. It also helps to have internal projects break first.

[danielsn]

A few questions but LGTM