Fix subject embedding in frame algorithm.

- General improvements/simplifications w/frame algorithm.
- Remove extra _embedValues call with simpler recursion.
- Add subjectStack to check for circular references properly;
  previously the unique embed map was used which may yield
  false positives and not embed where there actually wasn't
  a circular reference.
- Create implicit frames for properties in frames that don't
  have explicit subframes; this allows for cleaner recursion,
  less repititous code and ensures embeds occur whenever possible
  when using `@embed`: `@always`.
- Use `@last` instead of `@once` for `@embed` as it is more clear
  about how the embed will occur (last wins). `@once` is still
  backwards-compat supported for now.
Test to ensure a non-circular recursive property receives an embed.
Release 0.3.9.
Start 0.3.10.