Remove equivocating validators from fork choice (#3371)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/3241
Closes https://github.com/sigp/lighthouse/issues/3242

## Proposed Changes

* [x] Implement logic to remove equivocating validators from fork choice per https://github.com/ethereum/consensus-specs/pull/2845
* [x] Update tests to v1.2.0-rc.1. The new test which exercises `equivocating_indices` is passing.
* [x] Pull in some SSZ abstractions from the `tree-states` branch that make implementing Vec-compatible encoding for types like `BTreeSet` and `BTreeMap`.
* [x] Implement schema upgrades and downgrades for the database (new schema version is V11).
* [x] Apply attester slashings from blocks to fork choice

## Additional Info

* This PR doesn't need the `BTreeMap` impl, but `tree-states` does, and I don't think there's any harm in keeping it. But I could also be convinced to drop it.

Blocked on #3322.

consensus/ssz/src/decode/try_from_iter.rs

@@ -0,0 +1,96 @@
+use smallvec::SmallVec;
+use std::collections::{BTreeMap, BTreeSet};
+use std::convert::Infallible;
+use std::fmt::Debug;
+
+/// Partial variant of `std::iter::FromIterator`.
+///
+/// This trait is implemented for types which can be constructed from an iterator of decoded SSZ
+/// values, but which may refuse values once a length limit is reached.
+pub trait TryFromIter<T>: Sized {
+    type Error: Debug;
+
+    fn try_from_iter<I>(iter: I) -> Result<Self, Self::Error>
+    where
+        I: IntoIterator<Item = T>;
+}
+
+// It would be nice to be able to do a blanket impl, e.g.
+//
+// `impl TryFromIter<T> for C where C: FromIterator<T>`
+//
+// However this runs into trait coherence issues due to the type parameter `T` on `TryFromIter`.
+//
+// E.g. If we added an impl downstream for `List<T, N>` then another crate downstream of that
+// could legally add an impl of `FromIterator<Local> for List<Local, N>` which would create
+// two conflicting implementations for `List<Local, N>`. Hence the `List<T, N>` impl is disallowed
+// by the compiler in the presence of the blanket impl. That's obviously annoying, so we opt to
+// abandon the blanket impl in favour of impls for selected types.
+impl<T> TryFromIter<T> for Vec<T> {
+    type Error = Infallible;
+
+    fn try_from_iter<I>(iter: I) -> Result<Self, Self::Error>
+    where
+        I: IntoIterator<Item = T>,
+    {
+        Ok(Self::from_iter(iter))
+    }
+}
+
+impl<T, const N: usize> TryFromIter<T> for SmallVec<[T; N]> {
+    type Error = Infallible;
+
+    fn try_from_iter<I>(iter: I) -> Result<Self, Self::Error>
+    where
+        I: IntoIterator<Item = T>,
+    {
+        Ok(Self::from_iter(iter))
+    }
+}
+
+impl<K, V> TryFromIter<(K, V)> for BTreeMap<K, V>
+where
+    K: Ord,
+{
+    type Error = Infallible;
+
+    fn try_from_iter<I>(iter: I) -> Result<Self, Self::Error>
+    where
+        I: IntoIterator<Item = (K, V)>,
+    {
+        Ok(Self::from_iter(iter))
+    }
+}
+
+impl<T> TryFromIter<T> for BTreeSet<T>
+where
+    T: Ord,
+{
+    type Error = Infallible;
+
+    fn try_from_iter<I>(iter: I) -> Result<Self, Self::Error>
+    where
+        I: IntoIterator<Item = T>,
+    {
+        Ok(Self::from_iter(iter))
+    }
+}
+
+/// Partial variant of `collect`.
+pub trait TryCollect: Iterator {
+    fn try_collect<C>(self) -> Result<C, C::Error>
+    where
+        C: TryFromIter<Self::Item>;
+}
+
+impl<I> TryCollect for I
+where
+    I: Iterator,
+{
+    fn try_collect<C>(self) -> Result<C, C::Error>
+    where
+        C: TryFromIter<Self::Item>,
+    {
+        C::try_from_iter(self)
+    }
+}