Dynamically allocate contact and efc_ arrays on a new memory arena.

- Add private function `mj_arenaAlloc`. This is used internally to allocate memory from the arena.

- Add private function `mj_nefc` to count constraints. This function returns a tight upper bound on `d->nefc`. The number of counted constraints can be slightly bigger than exact `d->nefc` in the case of constraints with empty Jacobian, as when placing a frictional tendon between two world sites.

- Add new `memory` attribute to the `size` XML element for specification of arena memory size. This attribute is mutually exclusive with `nstack` and `njmax` specifications, which are now deprecated (but left around for the time being for legacy compatibility).

- Move `d->stack` to the end of the new arena space. The stack now grows in reverse from the end.

PiperOrigin-RevId: 479341539
Change-Id: Ie019c202e0908577ffc6f833a37920858116f667

src/engine/engine_collision_driver.c

@@ -19,6 +19,7 @@
 
 #include <mujoco/mjdata.h>
 #include <mujoco/mjmodel.h>
+#include <mujoco/mjxmacro.h>
 #include "engine/engine_callback.h"
 #include "engine/engine_collision_convex.h"
 #include "engine/engine_collision_primitive.h"
@@ -55,7 +56,7 @@ void mj_collision(const mjModel* m, mjData* d) {
   int *broadphasepair = 0;
   mjMARKSTACK;
 
-  // clear size
+  // reset the size of the contact array
   d->ncon = 0;
 
   // return if disabled