Cache array length in rb_ary_join (#15362)

When all elements are strings, we never have to recalculate the length
of the array because there are no conversion methods that are called, so
the length will never change. This speeds up the fast path by ~10%.

```ruby
a = ["1"*10, "2"*10, "3"*10, "4"*10, "5"*10] * 10
10_000_000.times do
  a.join
end
```

```
hyperfine --warmup 1 'ruby ../ruby2/test.rb' './exe/ruby ../ruby2/test.rb'
Benchmark 1: ruby ../ruby2/test.rb
  Time (mean ± σ):      3.779 s ±  0.053 s    [User: 3.754 s, System: 0.017 s]
  Range (min … max):    3.715 s …  3.874 s    10 runs

Benchmark 2: ./exe/ruby ../ruby2/test.rb
  Time (mean ± σ):      3.411 s ±  0.038 s    [User: 3.387 s, System: 0.017 s]
  Range (min … max):    3.360 s …  3.472 s    10 runs

Summary
  ./exe/ruby ../ruby2/test.rb ran
    1.11 ± 0.02 times faster than ruby ../ruby2/test.rb
```

array.c

@@ -2917,23 +2917,28 @@ rb_ary_join(VALUE ary, VALUE sep)
         StringValue(sep);
         len += RSTRING_LEN(sep) * (RARRAY_LEN(ary) - 1);
     }
-    for (i=0; i<RARRAY_LEN(ary); i++) {
+    long len_memo = RARRAY_LEN(ary);
+    for (i=0; i < len_memo; i++) {
         val = RARRAY_AREF(ary, i);
-        tmp = rb_check_string_type(val);
-
-        if (NIL_P(tmp) || tmp != val) {
-            int first;
-            long n = RARRAY_LEN(ary);
-            if (i > n) i = n;
-            result = rb_str_buf_new(len + (n-i)*10);
-            rb_enc_associate(result, rb_usascii_encoding());
-            i = ary_join_0(ary, sep, i, result);
-            first = i == 0;
-            ary_join_1(ary, ary, sep, i, result, &first);
-            return result;
+        if (RB_UNLIKELY(!RB_TYPE_P(val, T_STRING))) {
+            tmp = rb_check_string_type(val);
+            if (NIL_P(tmp) || tmp != val) {
+                int first;
+                long n = RARRAY_LEN(ary);
+                if (i > n) i = n;
+                result = rb_str_buf_new(len + (n-i)*10);
+                rb_enc_associate(result, rb_usascii_encoding());
+                i = ary_join_0(ary, sep, i, result);
+                first = i == 0;
+                ary_join_1(ary, ary, sep, i, result, &first);
+                return result;
+            }
+            len += RSTRING_LEN(tmp);
+            len_memo = RARRAY_LEN(ary);
+        }
+        else {
+            len += RSTRING_LEN(val);
         }
-
-        len += RSTRING_LEN(tmp);
     }
 
     result = rb_str_new(0, len);