Implement first class functions

[daniel-p-gonzalez]

Functions should be treated as first class objects.
e.g.

def sqr(x):
  return x * x
end

def test(function):
  print(function(5)) # 25
end

def main():
  print(sqr(5)) # 25
  fun = sqr
  fun(5) # 25
  test(fun)
end