EVM Engineering: make Slither useful again

[smartcontracts]

We're planning to drop Slither from GitHub actions for now. Slither action hasn't been useful for a long time. We'd like to reintroduce Slither with the following modifications:

• Slither runs in CI for new PRs, only looks at the PR diff, reports findings, and requires PRs to address the findings
• Slither runs in CI on merge to develop and scans the entire repository

All of this is similar to semgrep.

Should be done in the Circle CI workflow instead of in GitHub actions.