Fix some bugs

[NoahStoryM]

Bug 1:

> (run* (q) (== q #f))
'()

In the definition of case-inf, the cond's first branch:

[(not a-inf) e0]

When q was bound to #f, however, #f was regard as a kind of failure, then it was ignored.

So instead of putting the result directly into stream, it's better to encapsulate it with a list first.

Bug 2:

> (run 1.2 (q) (== q #t))
'(#t)
> (run -3 (q) (== q #t))
'(#t)

take should not accept negative number or float number.

Bug 3:

> (define teacupo
    (λ (x)
      (conde [(== 'tea x) succeed]
             [(== 'cup x) succeed]
             [fail])))
> (run* (r)
        (conde [(teacupo r) succeed]
               [(== 'f r) succeed]
               [fail]))
'(f tea cup)

It should return '(tea cup #f) .

On the Appendix A of The Reasoned Schemer, mplus and mplusi are defined like this:

(define mplus
  (λ (a-inf f)
    (case-inf a-inf
      (f)
      [(a) (choice a f)]
      [(a f0) (choice a (λf@ () (mplus (f0) f)))])))

(define mplusi
  (λ (a-inf f)
    (case-inf a-inf
      (f)
      [(a) (choice a f)]
      [(a f0) (choice a (λf@ () (mplusi (f) f0)))])))

We can see that mplusm is defined as mplusi instead of mplus in the old code.