## Issue Addressed
Fixes#2141
Remove [tempdir](https://docs.rs/tempdir/0.3.7/tempdir/) in favor of [tempfile](https://docs.rs/tempfile/3.1.0/tempfile/).
## Proposed Changes
`tempfile` has a slightly different api that makes creating temp folders with a name prefix a chore (`tempdir::TempDir::new("toto")` => `tempfile::Builder::new().prefix("toto").tempdir()`).
So I removed temp folder name prefix where I deemed it not useful.
Otherwise, the functionality is the same.
This is an implementation of a slasher that lives inside the BN and can be enabled via `lighthouse bn --slasher`.
Features included in this PR:
- [x] Detection of attester slashing conditions (double votes, surrounds existing, surrounded by existing)
- [x] Integration into Lighthouse's attestation verification flow
- [x] Detection of proposer slashing conditions
- [x] Extraction of attestations from blocks as they are verified
- [x] Compression of chunks
- [x] Configurable history length
- [x] Pruning of old attestations and blocks
- [x] More tests
Future work:
* Focus on a slice of history separate from the most recent N epochs (e.g. epochs `current - K` to `current - M`)
* Run out-of-process
* Ingest attestations from the chain without a resync
Design notes are here https://hackmd.io/@sproul/HJSEklmPL
