Optimize Range#bsearch for beginless/endless ranges

On Range#bsearch for endless ranges, we try positions at `begin + 2**i` (i = 0, 1, 2, ...)
to find a point that satisfies a given condition.
Subsequently, we perform binary searching with the interval `[begin, begin + 2**n]`.

However, the interval `[begin + 2**(n-1), begin + 2**n]` is sufficient for binary search
because `begin + 2**(n-1)` does not satisfy the condition.

The same applies to beginless ranges.

benchmark/range_bsearch.yml

@@ -0,0 +1,9 @@
+prelude: |
+  r = (1..)
+
+benchmark:
+  '10**1': r.bsearch { |x| x >= 10 }
+  '10**2': r.bsearch { |x| x >= 100 }
+  '10**3': r.bsearch { |x| x >= 1000 }
+  '10**4': r.bsearch { |x| x >= 10000 }
+  '10**5': r.bsearch { |x| x >= 100000 }

range.c

@@ -758,6 +758,7 @@ range_bsearch(VALUE range)
                 return bsearch_integer_range(beg, mid, 0);
             }
             diff = rb_funcall(diff, '*', 1, LONG2FIX(2));
+            beg = mid;
         }
     }
     else if (NIL_P(beg) && is_integer_p(end)) {
@@ -770,6 +771,7 @@ range_bsearch(VALUE range)
                 return bsearch_integer_range(mid, end, 0);
             }
             diff = rb_funcall(diff, '*', 1, LONG2FIX(2));
+            end = mid;
         }
     }
     else {