Move Reactor type definitions to Reactor.hpp.

Move Reactor type definitions to Reactor.hpp.

Just a refactoring, to make it clearer that Reactor types are the
higher level abstraction, with Nucleus the intermediate interface.

Bug swiftshader:10

Change-Id: I87d8a03cd05d7847511692c5ac9862e77d670801
Reviewed-on: https://swiftshader-review.googlesource.com/7279
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>