Interators, Generators, List-Comprehensions and For-Loops

[cpfr]

This pull request adds the "swiss army knife of iteration"[quote by Martin Odersky] to the Monty compiler:

• adding Iterable<Type> and Iterator<Type> classes to the core library
• adding Maybe<Type> to the core library

• adding for loops of the form (expresion must be of type Iterable<Type>)

  for i in expression:
    println(i)

• adding Generator functions

  • those can be implemented like funktions, but may use the yield statement instead of return
  • after yielding an expression, the execution of the generator is paused.
  • when the next element is requested, the execution resumes where it stopped previously
  • generator names use class identifiers, since they introduce a new iterable type

  generator Int Range(Int start, Int end):
    while start <= end:
        yield start
        start += 1
  
  for i in Range(1,3):  // [1,2,3]
    println(i)

  • (the example is simplified, since this does not work if start > end)
• adding support for the range literal, described in the specification [x..y], which is syntactic sugar for Range(x,y)
• adding list comprehensions of the form [Int x | x in [1..5] if x % 2 = 0], implemented as generators