Add Reference Copying

[nisdas]

This allows us to copy references to the objects instead of the actual objects. Doing this will reduce the required amount of memory when copying state fields. This is gated behind a feature flag so this doesn't affect production nodes in case of a bug.

• Add reference copying for fields
• Add feature flag
• Change method signature of ApplyToEveryValidator to take in two different functions
  so as to prevent unecessary clones.
• Use memory pools correctly for tries.
• Tests to ensure mutation is not possible
  • Validators (update + apply to all validators methods)
  • Block Roots
  • State Roots
  • Pending and Current Attestations
  • RANDAO

[prestonvanloon]

Discussed offline, please add some tests for these methods to validate and ensure mutation is not possible using this library. Especially the pointer slices like validators and pending attestations.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.