Issue 560 compute stack limit (string-append and bytevector-append)

[yorickhardy]

These commits attempt to address 2 issues:

1. When checking the available stack space for allocation, use the total size of the allocation instead of string or bytevector (or vector) lengths.
2. Rework string-append and bytevector-append to use alloc_string and alloc_bytevector so that the results are now correctly allocated on the stack or heap.

Tested in icyc with:

;; loops past the problem points in issue 560
(define s "1234567890")
(let loop ((a s))
   (display (string-length a))
   (newline)
   (loop (string-append a s)))

;; loops past the problem points in issue 560
(define s #u8(1 2 3 4 5 6 7 8 9 0))
(let loop ((a s))
  (display (bytevector-length a))
  (newline)
  (loop (bytevector-append a s)))

;; fails at length 147830
(define s #(1 2 3 4 5 6 7 8 9 0))
  (let loop ((a s))
    (display (vector-length a))
    (newline)
    (loop (vector-append a s)))