Address clippy lints in tree-states

[paulhauner]

Issue Addressed

NA

Proposed Changes

Most fixes are fairly straight-forward clippy changes, except for:

1. There was a test using BeaconStateDiff, which I couldn't find a definition for.
2. Implementing Default for Validator. It looks like it was purposefully disabled and I'm not sure why.

Additional Info

NA

[paulhauner]

When running make udeps I found the following compilation error:

error[E0599]: no method named `get_latest_restore_point_slot` found for struct `std::sync::Arc<store::HotColdDB<types::MinimalEthSpec, LevelDB<types::MinimalEthSpec>, LevelDB<types::MinimalEthSpec>>>` in the current scope
   --> beacon_node/beacon_chain/tests/store_tests.rs:403:41
    |
403 |     let last_restore_point_slot = store.get_latest_restore_point_slot();
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `Arc<HotColdDB<MinimalEthSpec, LevelDB<MinimalEthSpec>, LevelDB<MinimalEthSpec>>>`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `beacon_chain` due to previous error
warning: build failed, waiting for other jobs to finish...
make: *** [udeps] Error 101

get_latest_restore_point_slot has been removed. There's another test which has swapped that function for get_split_slot, it's accompanied by a // FIXME(sproul): consider whether this is 100% correct. I've just left the test as-is (i.e. not compiling) for now.

[michaelsproul]

Yeah sounds good. I'm planning to go through the tests and get them all compiling & passing again in the wake of the DB overhaul.

I think this PR is a nice improvement to the status quo, we've got clippy again! 🎉

Will merge once clippy passes