Add attester slashing support to block processing

At spec v0.2.0

eth2/state_processing/src/block_processable/verify_slashable_attestation.rs

@@ -0,0 +1,63 @@
+use super::Error;
+use log::error;
+use types::*;
+
+macro_rules! ensure {
+    ($condition: expr, $result: expr) => {
+        if !$condition {
+            return Err($result);
+        }
+    };
+}
+
+pub fn verify_slashable_attestation(
+    state: &mut BeaconState,
+    attester_slashing: &AttesterSlashing,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    let slashable_attestation_1 = &attester_slashing.slashable_attestation_1;
+    let slashable_attestation_2 = &attester_slashing.slashable_attestation_2;
+
+    ensure!(
+        slashable_attestation_1.data != slashable_attestation_2.data,
+        Error::BadAttesterSlashing
+    );
+    ensure!(
+        slashable_attestation_1.is_double_vote(slashable_attestation_2, spec)
+            | slashable_attestation_1.is_surround_vote(slashable_attestation_2, spec),
+        Error::BadAttesterSlashing
+    );
+    error!("this a");
+    ensure!(
+        state.verify_slashable_attestation(&slashable_attestation_1, spec),
+        Error::BadAttesterSlashing
+    );
+    error!("this b");
+    ensure!(
+        state.verify_slashable_attestation(&slashable_attestation_2, spec),
+        Error::BadAttesterSlashing
+    );
+    error!("this c");
+
+    let mut slashable_indices = vec![];
+    for i in &slashable_attestation_1.validator_indices {
+        let validator = state
+            .validator_registry
+            .get(*i as usize)
+            .ok_or_else(|| Error::BadAttesterSlashing)?;
+
+        if slashable_attestation_1.validator_indices.contains(&i)
+            & !validator.is_penalized_at(state.current_epoch(spec))
+        {
+            slashable_indices.push(i);
+        }
+    }
+
+    ensure!(slashable_indices.len() >= 1, Error::BadAttesterSlashing);
+
+    for i in slashable_indices {
+        state.penalize_validator(*i as usize, spec)?;
+    }
+
+    Ok(())
+}