py/vm: When VM raises exception put exc obj at beginning of func state.

Instead of at end of state, n_state - 1.  It was originally (way back in
v1.0) put at the end of the state because the VM didn't have a pointer to
the start.  But now that the VM takes a mp_code_state_t pointer it does
have a pointer to the start of the state so can put the exception object
there.

This commit saves about 30 bytes of code on all architectures, and, more
importantly, reduces C-stack usage by a couple of words (8 bytes on Thumb2
and 16 bytes on x86-64) for every (non-generator) call of a bytecode
function because fun_bc_call no longer needs to remember the n_state
variable.

py/objfun.c

@@ -318,7 +318,7 @@ STATIC mp_obj_t fun_bc_call(mp_obj_t self_in, size_t n_args, size_t n_kw, const
         // must be an exception because normal functions can't yield
         assert(vm_return_kind == MP_VM_RETURN_EXCEPTION);
         // return value is in fastn[0]==state[n_state - 1]
-        result = code_state->state[n_state - 1];
+        result = code_state->state[0];
     }
 
     #if MICROPY_ENABLE_PYSTACK

py/objgenerator.c

@@ -140,9 +140,8 @@ mp_vm_return_kind_t mp_obj_gen_resume(mp_obj_t self_in, mp_obj_t send_value, mp_
             break;
 
         case MP_VM_RETURN_EXCEPTION: {
-            size_t n_state = mp_decode_uint_value(self->code_state.fun_bc->bytecode);
             self->code_state.ip = 0;
-            *ret_val = self->code_state.state[n_state - 1];
+            *ret_val = self->code_state.state[0];
             // PEP479: if StopIteration is raised inside a generator it is replaced with RuntimeError
             if (mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(mp_obj_get_type(*ret_val)), MP_OBJ_FROM_PTR(&mp_type_StopIteration))) {
                 *ret_val = mp_obj_new_exception_msg(&mp_type_RuntimeError, "generator raised StopIteration");

py/vm.c

@@ -1468,7 +1468,7 @@ unwind_loop:
             } else {
                 // propagate exception to higher level
                 // Note: ip and sp don't have usable values at this point
-                fastn[0] = MP_OBJ_FROM_PTR(nlr.ret_val); // must put exception here because sp is invalid
+                code_state->state[0] = MP_OBJ_FROM_PTR(nlr.ret_val); // put exception here because sp is invalid
                 return MP_VM_RETURN_EXCEPTION;
             }
         }