Polkadot Protocol Specification v0.2.1

Now with exportable parser definition for blocks and runtime metadata.

• New runtime metadata chapter incl. full kaitai spec
• Added kaitai spec for basic block structure
• Introduce asciidoctor-kaitai to embed parsing definiton in spec
• Added testsuite to test exported parser against binary data
• Retired change-trie chapter
• Various improvements and corrections

For more details please see the full changelog.