Pre-allocate IO.select result arrays based on input size (ruby#15850)

io.c: pre-allocate IO.select result arrays based on input size

The ternary (rp?rb_ary_new():rb_ary_new2(0)) became pointless after
commit a51f30c (Variable Width Allocation, Mar 2022) made both
rb_ary_new() and rb_ary_new2(0) equivalent.

Instead of just removing the dead code, improve on the original intent
by pre-allocating based on the actual input array size. This avoids
reallocations when many FDs are ready.

Benchmark (100 ready FDs): ~8% improvement (5.59 -> 5.11 us/op)

Author: khasinski

io.c

@@ -10758,9 +10758,9 @@ select_internal(VALUE read, VALUE write, VALUE except, struct timeval *tp, rb_fd
     if (!pending && n == 0) return Qnil; /* returns nil on timeout */
 
     res = rb_ary_new2(3);
-    rb_ary_push(res, rp?rb_ary_new():rb_ary_new2(0));
-    rb_ary_push(res, wp?rb_ary_new():rb_ary_new2(0));
-    rb_ary_push(res, ep?rb_ary_new():rb_ary_new2(0));
+    rb_ary_push(res, rp ? rb_ary_new_capa(RARRAY_LEN(read)) : rb_ary_new());
+    rb_ary_push(res, wp ? rb_ary_new_capa(RARRAY_LEN(write)) : rb_ary_new());
+    rb_ary_push(res, ep ? rb_ary_new_capa(RARRAY_LEN(except)) : rb_ary_new());
 
     if (rp) {
         list = RARRAY_AREF(res, 0);