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/user/user_model.h

@@ -148,10 +148,11 @@ class mjCModel {
   std::string modelname;          // model name
   mjOption option;                // options
   mjVisual visual;                // visual options
+  std::size_t memory;             // size of arena+stack memory in bytes
   int nemax;                      // max number of equality constraints
   int njmax;                      // max number of constraints (Jacobian rows)
   int nconmax;                    // max number of detected contacts (mjContact array size)
-  int nstack;                     // number of fields in mjData stack
+  int nstack;                     // (deprecated) number of fields in mjData stack
   int nuserdata;                  // number extra fields in mjData
   int nuser_body;                 // number of mjtNums in body_user
   int nuser_jnt;                  // number of mjtNums in jnt_user