Add break back to the language

[elliottt]

As breakOut is considered unsafe in some contexts, e.g. in conjunction with a use of forever in the body of a tower task, it seems reasonable to consider this an effect to be tracked, rather than removing it from the language. If it's an effect, than it's still something that can be used by programmers, but restricted by API designers.

Defining an effect for break would yield a new signature for the breakOut function:

breakOut :: (CanBreak eff) => Ivory eff ()

Thus registering the requirement for a context that allows the breakOut function to be used. This has an added side-benefit of making it impossible to write functions where breakOut is used in a nonsensical position, as presumably only looping functions would introduce the CanBreak effect locally.

WIth this change to breakOut, the tower task loop example changes so that the argument that it will pass to forever has a very restricted context:

taskBody :: (outer `AllocsIn` s)
         => (forall inner. (inner `AllocsIn` s) => Ivory inner ())
         -> Ivory outer ()
taskBody body =
  do ...
     forever body

As you can see, the taskBody function takes an argument that only permits allocation effects, thus preventing uses of ret, and breakOut, as the new restrictive inner context will not permit them to be used.

[leepike]

Tracked by the type system now.