Implement mj_stackAlloc and mj_arenaAlloc alignment using bitwise-and.

This implementation explicitly avoids calling division instructions (which are generally expensive) even when link-time optimization is not active or if the alloc functions aren't inlined for some reason.

Also make mj_stackAlloc cheaper for external callers who may be linking against MuJoCo dynamically (and therefore cannot benefit from inlining).

PiperOrigin-RevId: 561299565
Change-Id: Iea4be5bf6d351ddf0c9d1fd8bd89a0c207b39309

doc/APIreference/functions.rst

@@ -1239,7 +1239,8 @@ mj_stackAlloc
 
 .. mujoco-include:: mj_stackAlloc
 
-Allocate a specific number of bytes on :ref:`mjData` stack. Call mju_error on stack overflow.
+Allocate a number of bytes on :ref:`mjData` stack at a specific alignment which must be a power of 2.
+Call mju_error on stack overflow.
 
 .. _mj_stackAllocNum: