Mixer's for non-primitive types

[mcgizzle]

The following will not compile

  @Atomic class A
  @Atomic class B
  implicitly[Mixer[(A,B), B]]

However if we swap the the classes for traits or primitives (String, Int, ect) it works fine.

See PR #20 for tests demonstrating the issue

[mcgizzle]

The failure comes from atomiseThenImpl and bIsAtomRecurse conflicting and causing ambiguous implicits.

Making bIsAtomRecurse lower priority solves the issue however it would be better to first understand why it is occurring first.