Added infix and prefix arithmetic operators.

Infix operators work by passing the corresponding prefix operator to the first operand, and then calling the result with the second operand.

For example:
- +[1, 2]
- -[3, 4]
- 1 + 3
- 4 - 2
- -(3 + 2)
- 3 / 2 + 1
- reciprocal = {1 / @x: @x}
- half = {2 * @x: @x}