Implement default function arguments (for Python functions).

TODO: Decide if we really need separate bytecode for creating functions
with default arguments - we would need same for closures, then there're
keywords arguments too. Having all combinations is a small exponential
explosion, likely we need just 2 cases - simplest (no defaults, no kw),
and full - defaults & kw.
diff --git a/py/showbc.c b/py/showbc.c
index 8a12302..53a1826 100644
--- a/py/showbc.c
+++ b/py/showbc.c
@@ -339,6 +339,11 @@
                 printf("MAKE_FUNCTION " UINT_FMT, unum);
                 break;
 
+            case MP_BC_MAKE_FUNCTION_DEFARGS:
+                DECODE_UINT;
+                printf("MAKE_FUNCTION_DEFARGS " UINT_FMT, unum);
+                break;
+
             case MP_BC_MAKE_CLOSURE:
                 DECODE_UINT;
                 printf("MAKE_CLOSURE " UINT_FMT, unum);