Block/header data structures

[asmie]

This task consists of the following steps:

• header data structures (according to chapter 5 of GP)
• block data structures
• serialization of the above (using Data items serialization #6 )

Header data structures are listed in Chapter 5 and then described in the next chapters.
Block data fields are described in the 4.1 sub-chapter and then described in the next chapters of Gray Paper, like, tickets in 6.7, verdicts in 10.2, availability in 11.2.1, reports 11.4, preimages in 12.1.

Some common types should be set-up to be used repo-wide: epochs, time, tickets etc.

[danielvladco]

Bandersnatch data types:
Signature -> https://github.com/paritytech/polkadot-sdk/blob/master/substrate/primitives/core/src/bandersnatch.rs#L68

• needed for vrf signature and the block seal

Block Author Key -> https://github.com/paritytech/polkadot-sdk/blob/master/substrate/primitives/core/src/bandersnatch.rs#L79

• (Not sure if this is the correct key type yet)

[asmie]

Header structures moved to a separate task: #15

[asmie]

Block data structures moved to a separate task: #17