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_io.h

@@ -96,6 +96,9 @@ MJAPI void mj_resetDataDebug(const mjModel* m, mjData* d, unsigned char debug_va
 // reset data, set fields from specified keyframe
 MJAPI void mj_resetDataKeyframe(const mjModel* m, mjData* d, int key);
 
+// mjData arena allocate
+void* mj_arenaAlloc(mjData* d, int bytes, int alignment);
+
 // mjData stack allocate
 MJAPI mjtNum* mj_stackAlloc(mjData* d, int size);