Implement proper exception type hierarchy.

Each built-in exception is now a type, with base type BaseException.
C exceptions are created by passing a pointer to the exception type to
make an instance of.  When raising an exception from the VM, an
instance is created automatically if an exception type is raised (as
opposed to an exception instance).

Exception matching (RT_BINARY_OP_EXCEPTION_MATCH) is now proper.

Handling of parse error changed to match new exceptions.

mp_const_type renamed to mp_type_type for consistency.

py/objgetitemiter.c

@@ -26,7 +26,7 @@ STATIC mp_obj_t it_iternext(mp_obj_t self_in) {
         return value;
     } else {
         // an exception was raised
-        if (MP_OBJ_IS_TYPE(nlr.ret_val, &exception_type) && mp_obj_exception_get_type(nlr.ret_val) == MP_QSTR_StopIteration) {
+        if (mp_obj_get_type(nlr.ret_val) == &mp_type_StopIteration) {
             // return mp_const_stop_iteration instead of raising StopIteration
             return mp_const_stop_iteration;
         } else {
@@ -37,7 +37,7 @@ STATIC mp_obj_t it_iternext(mp_obj_t self_in) {
 }
 
 STATIC const mp_obj_type_t it_type = {
-    { &mp_const_type },
+    { &mp_type_type },
     .name = MP_QSTR_iterator,
     .iternext = it_iternext
 };