adds binding of Automerge text on any invocation of encode

[heckj]

• exposes a new bind() method on AutomergeText that takes an ObjId for convenience

• switches AutomergeText.CodingKeys to public

• special cases encoding just an instance of AutomergeText into an AutomergeDocument

  • now correctly encoding the contents as .Text() instead of as a string Value
  • updated special case decode logic to accomodate the same, allowing it to be the top-level type stored into an Automerge document.

• fixes #145

• fixes #100