v0.2.0

This release includes a ton of new features, and a few breaking changes in various interfaces.
This version is also the first released to crates.io!

• Change: when a query fails because of a bad viewing key, this now correctly fails with StdError::Unauthorized
• Added support for some missing SNIP-20 functionality, such as CreateViewingKey
• Added support for SNIP-21 queries (memos and improved history) which broke some interfaces
• Added support for SNIP-22 messages (batch operations)
• Added support for SNIP-23 messages (improved Send operations) which broke some interfaces
• Added support for SNIP-24 permits
• Added Base64Of<S: Serde, T>, Base64JsonOf<T>, and Base64Bincode2Of<T>,
  which are wrappers that automatically deserializes base64 strings to T.
  It can be used in message types' fields instead of Binary when the contents of the string
  should have more specific contents.
• Added storage::DequeStore - Similar to AppendStore but allows pushing and popping on both ends
• Added the secret-toolkit::incubator package intended for experimental features. It contains:
  • CashMap - A hashmap like storage abstraction
  • GenerationalIndex - A generational index storage abstraction
• The various subpackages can now be selected using feature flags. The default flags are ["serialization", "snip20", "snip721", "storage", "utils"]
  while ["crypto", "permit", "incubator"] are left disabled by default.