py: Store bytecode arg names in bytecode (were in own array).

This saves a lot of RAM for 2 reasons:

1. For functions that don't have default values, var args or var kw
args (which is a large number of functions in the general case), the
mp_obj_fun_bc_t type now fits in 1 GC block (previously needed 2 because
of the extra pointer to point to the arg_names array).  So this saves 16
bytes per function (32 bytes on 64-bit machines).

2. Combining separate memory regions generally saves RAM because the
unused bytes at the end of the GC block are saved for 1 of the blocks
(since that block doesn't exist on its own anymore).  So generally this
saves 8 bytes per function.

Tested by importing lots of modules:

- 64-bit Linux gave about an 8% RAM saving for 86k of used RAM.
- pyboard gave about a 6% RAM saving for 31k of used RAM.

py/objfun.h

@@ -27,14 +27,13 @@
 typedef struct _mp_obj_fun_bc_t {
     mp_obj_base_t base;
     mp_obj_dict_t *globals;         // the context within which this function was defined
-    mp_uint_t n_pos_args : 16;      // number of arguments this function takes
-    mp_uint_t n_kwonly_args : 16;   // number of arguments this function takes
-    mp_uint_t n_def_args : 16;      // number of default arguments
+    mp_uint_t n_pos_args : 8;       // number of arguments this function takes
+    mp_uint_t n_kwonly_args : 8;    // number of keyword-only arguments this function takes
+    mp_uint_t n_def_args : 8;       // number of default arguments
     mp_uint_t has_def_kw_args : 1;  // set if this function has default keyword args
     mp_uint_t takes_var_args : 1;   // set if this function takes variable args
     mp_uint_t takes_kw_args : 1;    // set if this function takes keyword args
     const byte *bytecode;           // bytecode for the function
-    qstr *args;                     // argument names (needed to resolve positional args passed as keywords)
     // the following extra_args array is allocated space to take (in order):
     //  - values of positional default args (if any)
     //  - a single slot for default kw args dict (if it has them)