readint.c: add new function mrb_int_read.

Difference from `strtoul(3)`:

* reads `mrb_int` based on configuration
* specifies the end of the string
* no sign interpretation
* base 10 only

src/readint.c

@@ -0,0 +1,30 @@
+#include <mruby.h>
+#include <mruby/numeric.h>
+#include <errno.h>
+
+/* mrb_int_read(): read mrb_int from a string (base 10 only) */
+/* const char *p - string to read                            */
+/* const char *e - end of string                             */
+/* char **endp   - end of pased integer                      */
+
+/* if integer overflows, errno will be set to ERANGE         */
+/* also endp will be set to NULL on overflow                 */
+MRB_API mrb_int
+mrb_int_read(const char *p, const char *e, char **endp)
+{
+  mrb_int n = 0;
+  int ch;
+
+  while ((e == NULL || p < e) && ISDIGIT(*p)) {
+    ch = *p - '0';
+    if (mrb_int_mul_overflow(n, 10, &n) ||
+        mrb_int_add_overflow(n, ch, &n)) {
+      if (endp) *endp = NULL;
+      errno = ERANGE;
+      return MRB_INT_MAX;
+    }
+    p++;
+  }
+  if (endp) *endp = (char*)p;
+  return n;
+}