Fix #354, Added support for negating expressions

[EliasC]

Before we could write -1 (due to the parser library handling negation
of integers), but not -1.0 or -x or -1 + (-(1+2) + 3). This commit
adds support for arbitrary negation.

[PhucVH888]

I have checked with some more testcases such as tuple, pattern matching, and option type.
They are all passed.

class Main 
  def main() : void {
    let x = -(-42.0);
    let y = -(-(-x));
    let z = -y + (-(1+2) + 3);
    let t1 = match y with {
      -42.0   => print "-42.0"
      other => print "Nothing"
    };
    let t2 = match (y,y) with {
      (-42.0,-42.0)   => print "(-42.0,-42.0)"
      other => print "Nothing"
    };
    let mb1 = Just (-42.0);
    let t2 = match mb1 with {
      Just 42.0   => print "Just 42.0"
      Just something => print "Just something"
      --Just (-42.0)   => print "Just (-42.0)"
      Nothing => print "Nothing"
    };
    let f = \(i:real) -> {i = -9.0; print i};
    let rf = f(-9.0);
    print -z*2.0;
   }

[albertnetymk]

      print -3 - -1; -- -2
      print - -1; -- Unbound variable print
      let x = 3 in {print - - -x;}; -- unexpected "i"

The latter two cases don't work, but I think it's fine. Can be merged.