DO NOT MERGE (PR intendend for review comments): wormhole contracts

[ptrus]

WIP PR for internal reviews (before submitting this upstream).

• [ ] wormhole contract - somewhat done, TODOs:
  • [ ] implement gov_vaa_transfer_fee (emit accounts.Transfer)
  • [x] finish VAA signature verification (https://github.com/oasisprotocol/oasis-sdk/pull/495)
  • [x] tests (should probably add some more, but will do after finishing the rest)
  • [x] some cleanup of types/src/spec.rs (currently mostly copied and updated from upstream terra contracts - might require some cleanups)
• [ ] token bridge wormhole contract
  • [x] minor todo's
    • handle 8 decimals normalization/denormlaization
    • handle fees
  • [ ] tests

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For reviewers: suggested order:

• [ready for review] wormhole contract (in tests/contracts/wormhole/) - refs:
  • https://github.com/certusone/wormhole/blob/dev.v2/design/0001_generic_message_passing.md
  • https://github.com/certusone/wormhole/blob/dev.v2/design/0002_governance_messaging.md
  • https://github.com/certusone/wormhole/blob/dev.v2/design/0004_message_publishing.md
  • and the concrete implementations in the repository (since the specs are not all that specific)
• [ready for review] OAS20-wrapped contract (in tests/contracts/wormhole/)
  • OAS20 contract for wrapping tokens (owner can burn and mint tokens).
• [can also review, also not fully done yet] wormhole-token bridge contract (in tests/contracts/wormhole-token-bridge)
  • https://github.com/certusone/wormhole/blob/dev.v2/design/0003_token_bridge.md
  • and the concrete implementations in the repository (since the specs are not all that specific)