Make evaluate preserve vector immutability

[anishathalye]

Prior to this patch, the following returned an immutable vector:

(define empty-model (solve (void)))
(evaluate (vector-immutable 1) empty-model)

However, the following returned a mutable vector, even though evaluate is given an immutable vector (containing no symbolics):

(define-symbolic* b boolean?)
(define model (solve (assert b)))
(evaluate (vector-immutable 1) model)

With this patch, both examples above return an immutable vector.

[emina]

Thank you!