Fixed primitive.js to include assert so structs can be created.

[joshuacrotts]

In primitive.js, the assert function is used without defining it as a variable (and library function). As a result, defining structs does not work in the current version. For instance, the following code compiles without errors, but throws a sneaky runtime error.

(define-struct point [x y])
(point-x (make-point 5 10))

This patch fixes the issue and correctly outputs 5.