py: Rework and compress second part of bytecode prelude.

This patch compresses the second part of the bytecode prelude which
contains the source file name, function name, source-line-number mapping
and cell closure information.  This part of the prelude now begins with a
single varible length unsigned integer which encodes 2 numbers, being the
byte-size of the following 2 sections in the header: the "source info
section" and the "closure section".  After decoding this variable unsigned
integer it's possible to skip over one or both of these sections very
easily.

This scheme saves about 2 bytes for most functions compared to the original
format: one in the case that there are no closure cells, and one because
padding was eliminated.

py/objfun.c

@@ -139,7 +139,7 @@ const mp_obj_type_t mp_type_fun_builtin_var = {
 /* byte code functions                                                        */
 
 qstr mp_obj_code_get_name(const byte *code_info) {
-    code_info = mp_decode_uint_skip(code_info); // skip code_info_size entry
+    MP_BC_PRELUDE_SIZE_DECODE(code_info);
     #if MICROPY_PERSISTENT_CODE
     return code_info[0] | (code_info[1] << 8);
     #else