Allow lazy vals without await in the initializer

We were incorrectly typechecking the `ClassDef` of the state machine
in the macro in a way that discarded the resulting trees, and only
kept around the symbol.

The led to the the macro engine retypechecking
that node, which somehow led to duplicated lazy val initiaializer
`DefDef`-s in the template, which manifest as a `VerifyError`.

This commit:

  - rescues the typechecked `ClassDef` node from the eager
    typechecking by the macro
  - loosens the restriction on lazy vals in async blocks. They are
    still prohibited if they contain an await on the RHS
  - Adds a test that shows evalution is indeed lazy.

Fixes #52
Incorporate pull request feedback

  - remove unneeded `setType(NoType)`, which was leftover from my
    first attempts to find this bug.
  - fix typo in error message
  - optimize imports
Merge pull request #67 from retronym/ticket/52-lazy-val

Allow lazy vals without await in the initializer