Move state trans fns into state_processing

eth2/state_processing/src/per_epoch_processing/process_ejections.rs

@@ -0,0 +1,28 @@
+use crate::common::exit_validator;
+use types::{BeaconStateError as Error, *};
+
+/// Iterate through the validator registry and eject active validators with balance below
+/// ``EJECTION_BALANCE``.
+///
+/// Spec v0.5.0
+pub fn process_ejections(state: &mut BeaconState, spec: &ChainSpec) -> Result<(), Error> {
+    // There is an awkward double (triple?) loop here because we can't loop across the borrowed
+    // active validator indices and mutate state in the one loop.
+    let exitable: Vec<usize> = state
+        .get_active_validator_indices(state.current_epoch(spec), spec)?
+        .iter()
+        .filter_map(|&i| {
+            if state.validator_balances[i as usize] < spec.ejection_balance {
+                Some(i)
+            } else {
+                None
+            }
+        })
+        .collect();
+
+    for validator_index in exitable {
+        exit_validator(state, validator_index, spec)?
+    }
+
+    Ok(())
+}