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

introspect/functions.py

@@ -699,7 +699,7 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
                  type=ValueType(name='size_t'),
              ),
          ),
-         doc='Allocate a specific number of bytes on mjData stack. Call mju_error on stack overflow.',  # pylint: disable=line-too-long
+         doc='Allocate a number of bytes on mjData stack at a specific alignment which must be a power of 2. Call mju_error on stack overflow.',  # pylint: disable=line-too-long
      )),
     ('mj_stackAllocNum',
      FunctionDecl(