Use custom types in data structures and constants

[Nashatyrev]

Original issue: ethereum#667

The goal is to improve Spec readability by using alias custom types in data structures and as constant units instead of generic types.

What was done

• Add Hash and Bitfield custom types. The Hash type is treated as a result of any 256-bit cryptographic hash function or its derivative preserving hash properties (like xoring two hashes)
• Replace generic SSZ types in data structures with custom types where appropriate
• Add/update constant units according to custom types

What else can be done

• Add more custom types (like Time, ForkVersion, Bitfield64) which are now referred at 1-2 places in the Spec
• As per @hwwhww suggestion in Use custom types in data structures ethereum/consensus-specs#667 cover all elements with custom types
• In some way describe array index types (like 'validator_balances': [ValidatorIndex => Gwei])

[mkalinin]

LGTM! 👍