constant squishing operators

[obriencj]

allow compile-time operators to squish left-most constant values.

For example,

(+ 1 2 3) := 6
(+ a 2 3) := (+ a 2 3)  ;; no squish past the first non-constant
(+ 2 3 a) := (+ 5 a)

The build-string operator already does this.