Shutter: validator registry V1

[Marchhill]

Changes

• Implements V1 of validator registry spec (Use BLS aggregate signatures for validator registry gnosischain/specs#53)
• Refactor validator registry contact
• Add tests for validator registration check

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a change that causes existing functionality not to work as expected)
• Optimization
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Documentation

Requires documentation update

• Yes
• No

Requires explanation in Release Notes

• Yes
• No