max_frame_size.md

1. Extend BEGINSUB specification with third parameter max_frame_size -- the maximum size of the stack frame the subroutine is allowed to use.

2. In the Validity section, after condition 6 add new conditions:

   6a. For JUMPSUB and JUMPSUBV the frame size + the max_frame_size of the BEGINSUB(s) to jump to is not greater than 1024.

   6b. The frame size is not greater than the max_frame_size of the enclosing subroutine.

3. In the Validating subroutines section, after

   // check for stack overflow
   if SP > 1024
       return false
   

   add

   // check for exiding the max_frame_size
   if SP + max_frame_size(return_pc) > 1024
       return false