py: Split RAISE_VARARGS opcode into 3 separate ones.

From the beginning of this project the RAISE_VARARGS opcode was named and
implemented following CPython, where it has an argument (to the opcode)
counting how many args the raise takes:

    raise # 0 args (re-raise previous exception)
    raise exc # 1 arg
    raise exc from exc2 # 2 args (chained raise)

In the bytecode this operation therefore takes 2 bytes, one for
RAISE_VARARGS and one for the number of args.

This patch splits this opcode into 3, where each is now a single byte.
This reduces bytecode size by 1 byte for each use of raise.  Every byte
counts!  It also has the benefit of reducing code size (on all ports except
nanbox).

py/showbc.c

@@ -500,9 +500,16 @@ const byte *mp_bytecode_print_str(const byte *ip) {
             printf("RETURN_VALUE");
             break;
 
-        case MP_BC_RAISE_VARARGS:
-            unum = *ip++;
-            printf("RAISE_VARARGS " UINT_FMT, unum);
+        case MP_BC_RAISE_LAST:
+            printf("RAISE_LAST");
+            break;
+
+        case MP_BC_RAISE_OBJ:
+            printf("RAISE_OBJ");
+            break;
+
+        case MP_BC_RAISE_FROM:
+            printf("RAISE_FROM");
             break;
 
         case MP_BC_YIELD_VALUE: