Add serde_utils module with quoted u64 support (#1588)

## Proposed Changes

This is an extraction of the quoted int code from #1569, that I've come to rely on for #1544.

It allows us to parse integers from serde strings in YAML, JSON, etc. The main differences from the code in Paul's original PR are:

* Added a submodule that makes quoting mandatory (`require_quotes`).
* Decoding is generic over the type `T` being decoded. You can use `#[serde(with = "serde_utils::quoted_u64::require_quotes")]` on `Epoch` and `Slot` fields (this is what I do in my slashing protection PR).

I've turned on quoting for `Epoch` and `Slot` in this PR, but will leave the other `types` changes to you Paul.

I opted to put everything in the `conseus/serde_utils` module so that BLS can use it without a circular dependency. In future when we want to publish `types` I think we could publish `serde_utils` as `lighthouse_serde_utils` or something. Open to other ideas on this front too.

consensus/types/src/slot_epoch.rs

@@ -25,11 +25,12 @@ use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Rem, Sub, SubAssi
 #[cfg_attr(feature = "arbitrary-fuzz", derive(arbitrary::Arbitrary))]
 #[derive(Eq, Clone, Copy, Default, Serialize, Deserialize)]
 #[serde(transparent)]
-pub struct Slot(u64);
+pub struct Slot(#[serde(with = "serde_utils::quoted_u64")] u64);
 
 #[cfg_attr(feature = "arbitrary-fuzz", derive(arbitrary::Arbitrary))]
 #[derive(Eq, Clone, Copy, Default, Serialize, Deserialize)]
-pub struct Epoch(u64);
+#[serde(transparent)]
+pub struct Epoch(#[serde(with = "serde_utils::quoted_u64")] u64);
 
 impl_common!(Slot);
 impl_common!(Epoch);

consensus/types/src/utils.rs

@@ -1,3 +1,3 @@
 mod serde_utils;
 
-pub use serde_utils::*;
+pub use self::serde_utils::*;