Upgrade cats (issue #212), optimize Task, refactorings

[alexandru]

• upgrade the Cats dependency to version 0.7.0, see issue Upgrade Cats dependency to 0.7 #212
• rename eval to evalAlways across the board (in Task, Coeval and Observable), but keep evalAlways with the @deprecated sign
• rename eval(Coeval) to coeval(Coeval) in Task and Observable
• refactor the Task internals again, for optimizations
  • simplifies the internal states, e.g. instead of having Now, Error, Always and Once, we now have a single Delay(coeval)
  • get rid of Task.Attempt, it never made any sense that one. People can use Coeval.Attempt if they need a Try alternative (and convert to Task if they end up needing a Task)
• moved everything from monix.types.shims to monix.types
• in monix.types, split the Deferrable type in Suspendable and Memoizable
• in monix-execution introduce executeAsync and executeLocal as macros
• use executeLocal and LocalRunnable in key points in the Task implementation to reduce forking