grow the tmps (mortal) stack exponentially rather than linearly

As with the value stack and the save stack, this gives us constant
amortized growth per element.

After this patch the profiler shows the "SvPV_shrink_to_cur(sv)"
and "sv = sv_2mortal(newSV(80))" calls in do_readline as the
hotspots for the io unheated test case, using 55% of the measured
time in total.

Fixes #21654

scope.c

@@ -246,8 +246,12 @@ Perl_tmps_grow_p(pTHX_ SSize_t ix)
 {
     SSize_t extend_to = ix;
 #ifndef STRESS_REALLOC
-    if (ix - PL_tmps_max < 128)
-        extend_to += (PL_tmps_max < 512) ? 128 : 512;
+    SSize_t grow_size = PL_tmps_max < 512 ? 128 : PL_tmps_max / 5;
+    if (extend_to >= SSize_t_MAX - grow_size)
+        /* trigger memwrap message or fail allocation */
+        extend_to = SSize_t_MAX-1;
+    else
+        extend_to += grow_size;
 #endif
     Renew(PL_tmps_stack, extend_to + 1, SV*);
     PL_tmps_max = extend_to + 1;

t/perf/tmps.t

@@ -48,7 +48,6 @@ my $min_small = min($basetimes{$small_size}->@*);
 my $min_large = min($basetimes{$large_size}->@*);
 my $ratio = $large_size / $small_size;
 
-our $TODO = "tmps stack grows in O(n**2) time";
 my $worst = $min_small * $ratio * 2;
 note "worst allowed $worst";
 cmp_ok($min_large, '<', $worst,