Refactorings across the board

[alexandru]

The monix-execution sub-project:

• introduce the CallbackRunnable interface for marking
  Runnable instances that could be executed on the current
  thread, on a local trampoline, as an optimization
• introduce LocalBatchingExecutor, a mixin for schedulers
  that can execute CallbackRunnable locally
• make AsyncScheduler on the JVM be able to execute
  CallbackRunnable instances by inheriting from
  LocalBatchingExecutor
• fixed critical bug in CompositeCancelable that
  was introduced in the last release

The monix-eval sub-project:

• optimized Task.fromFuture to the point that it has near zero overhead
• optimized Task.gather, Task.gatherUnordered, Task.sequence
• optimized the Task trampoline in general
• introduce Callback.async wrapper and asyncApply extension method
• renamed Task.zipWith to Task.zipMap (rename across the board)
• added Task.executeOn for overriding the Scheduler
• added Task.fork overload with a scheduler override
• added Task.async as an alias of Task.create

The monix-types sub-project:

• moved all shims to monix.types.shims
• add Deferrable type-class
• add Evaluable type-class

The monix-reactive project:

• for foldLeft methods, make the seed be just a normal
  by-name parameter instead of a Coeval, because otherwise
  it isn't compatible with other type-classes / interfaces
• affected methods are foldLeft, foldLeftAsync
• rename zipWith to zipMap
• rename combineLatestWith to combineLatestMap
• add Observable.fromAsyncStateAction
• fix Consumer.foldLeftAsync