Increase benchmark iterations to improve precision in comparisons over time

* benchmark-suite/benchmarks/continuations.bm: Increase "call/cc"
  iterations from 300 to 12000.

* benchmark-suite/benchmarks/srfi-13.bm ("strings"): Increase "copy"
  iterations from 1100 to 20000, and "pad" from 6800 to 34000.

* benchmark-suite/benchmarks/uniform-vector-read.bm
  ("uniform-vector-read!"): Increase "uniform-vector-write" iterations
  from 500 to 4000, and "uniform-vector-read!" from 500 to 20000.

benchmark-suite/benchmarks/continuations.bm

@@ -5,5 +5,5 @@
 (define (callee continuation)
   (continuation #t))
 
-(benchmark "call/cc" 300
+(benchmark "call/cc" 12000
   (call-with-current-continuation callee))

benchmark-suite/benchmarks/srfi-13.bm

@@ -122,7 +122,7 @@ Italiam, fato profugus, Laviniaque venit")
               (string-ref long-string k)
               (loop (+ k 1))))))
 
-    (benchmark "copy" 1100
+    (benchmark "copy" 20000
       (string-copy short-string)
       (string-copy medium-string)
       (string-copy long-string)
@@ -130,7 +130,7 @@ Italiam, fato profugus, Laviniaque venit")
       (substring/copy medium-string 10 20)
       (substring/copy long-string 100 200))
 
-    (benchmark "pad" 6800
+    (benchmark "pad" 34000
       (string-pad short-string 100)
       (string-pad medium-string 100)
       (string-pad long-string 100))

benchmark-suite/benchmarks/uniform-vector-read.bm

@@ -36,12 +36,12 @@
 
 (with-benchmark-prefix "uniform-vector-read!"
 
-  (benchmark "uniform-vector-write" 500
+  (benchmark "uniform-vector-write" 4000
     (let ((output (open-output-file file-name)))
       (uniform-vector-write buf output)
       (close output)))
 
-  (benchmark "uniform-vector-read!" 500
+  (benchmark "uniform-vector-read!" 20000
     (let ((input (open-input-file file-name)))
       (setvbuf input _IONBF)
       (uniform-vector-read! buf input)