## Proposed Changes
Update `Cross.toml` for the recently released Cross v0.2.2. This allows us to remove the dependency on my fork of the Cross Docker image, which was a maintenance burden and prone to bit-rot. This PR puts us back in sync with upstream Cross.
## Additional Info
Due to some bindgen errors on the default Cross images we seemingly need a full `clang-3.9` install. The `libclang-3.9-dev` package was found to be insufficient due to `stdarg.h` being missing.
In order to continue building locally all Lighthouse devs should update their local cross version with `cargo install cross`.
## Issue Addressed
Closes#2286Closes#2538Closes#2342
## Proposed Changes
Part II of major slasher optimisations after #2767
These changes will be backwards-incompatible due to the move to MDBX (and the schema change) 😱
* [x] Shrink attester keys from 16 bytes to 7 bytes.
* [x] Shrink attester records from 64 bytes to 6 bytes.
* [x] Separate `DiskConfig` from regular `Config`.
* [x] Add configuration for the LRU cache size.
* [x] Add a "migration" that deletes any legacy LMDB database.
## Issue Addressed
NA
## Proposed Changes
Tells `cross` (used for cross-compiling) to read the `RUSTFLAGS`env and pass it through during build. This allows us to use `-g` and get debug info.
## Additional Info
NA
