Put bytecode stack frame metadata in a struct

Put bytecode stack frame metadata in a struct

Using a plain C struct instead of type-punning Lisp_Object stack slots
makes the bytecode interpreter code more type-safe and potentially
faster (from better alias analysis), and the special-purpose accessors
are no longer needed.  It also reduces the stack requirements when
using 64-bit Lisp_Object on 32-bit platforms.

* src/bytecode.c (enum stack_frame_index)
(sf_get_ptr, sf_set_ptr, sf_get_lisp_ptr, sf_set_lisp_ptr,
sf_get_saved_pc, sf_set_saved_pc): Remove.
(BC_STACK_SIZE): Now in bytes, not Lisp words.
(struct bc_frame): New.
(init_bc_thread, mark_bytecode, Finternal_stack_stats, valid_sp)
(exec_byte_code):
* src/lisp.h (struct handler, get_act_rec, set_act_rec):
Adapt to new struct bc_frame.