syntax: while condition can now cast variable

Example:
  var a = foo # foo gives a A
  # Before the while, the base static type of a is set to A
  while a isa B do
    a.some_b_stuff # OK, since a isa B
    a = foo # OK, since 'base' static type of a is A
  end

Signed-off-by: Jean Privat <jean@pryen.org>