support in-place operators

[obriencj]

iadd, isub, imult, idiv, etc.

They should not be reducing in the fashion of the existing operators.
Create assigning macros for these which perform a final setf. eg:

(+= a 100)  :=  (setf a (iadd a 100))
(-= a 5) := (setf a (isub a 5))