add immutable-vector, immutable-vector-copy, etc.

[mflatt]

Based on a suggestion and initial work from @owaddell, extract the immutable-vector addition from #596, and then add immutable-vector-copy and friends. The motivation for the additions is the same as for vector-copy and company: to implement persistent data structures, but where you want to enforce immutability and maybe enable optimizations.

The changes include cp0 rules to combine vector-construction operations, such as turning (vector->immutable-vector (vector-append (vector x y) '#(3))) into (immutable-vector x y 3).

It turns out that (immutable-vector-copy x) and (immutable-vector-append x) are both equivalent to (vector->immutable-vector x). This PR doesn't try to prune the API, although it's possible that we would have left out vector->immutable-vector if immutable-vector-copy existed in the first place.

[burgerrg]

These procedures make immutable vectors much easier to use, thanks!

[owaddell]

I'd like to extend the inline handler for vector-append and immutable-vector-append slightly to handle the following:

(define (expand-segments segments)
  (let ([n (vector-length segments)])
    (immutable-vector-append segments (make-vector n #f))))

Right now cptypes doesn't recognize that n will be a safe argument for make-vector, so it doesn't switch to #3%make-vector. @gus-massa are you interested in helping refine cptypes to handle this?

Current work-in-progress patch to cpprims.ss: patch.txt

[mflatt]

This PR now pulls in @owaddell's improvements. There's no cptypes changes, yet, and those might be best considered in a new PR.

[mflatt]

More improvements from @owaddell now added.