Overload + operator to allow both int and real arguments

[julianhyde]

Overload + operator (and other arithmetic operators) to allow both int and real arguments. If there are no clues in the argument types, it should default to int. For example:

- fun plus x y = x + y;
val plus = fn : int -> int -> int
- fun plus2 x y = x + y + 0.0;
val plus2 = fn : real -> real -> real

[julianhyde]

Fixed in 3aafd3b.