Otherwise it can clash with time_t from the C standard include headers.

Header files that are considered internal to the py core and should not
normally be included directly are:
    py/nlr.h - internal nlr configuration and declarations
    py/bc0.h - contains bytecode macro definitions
    py/runtime0.h - contains basic runtime enums

Instead, the top-level header files to include are one of:
    py/obj.h - includes runtime0.h and defines everything to use the
        mp_obj_t type
    py/runtime.h - includes mpstate.h and hence nlr.h, obj.h, runtime0.h,
        and defines everything to use the general runtime support functions

Additional, specific headers (eg py/objlist.h) can be included if needed.

Builtin functions with a fixed number of arguments (0, 1, 2 or 3) are
quite common.  Before this patch the wrapper for such a function cost
3 machine words.  After this patch it only takes 2, which can reduce the
code size by quite a bit (and pays off even more, the more functions are
added).  It also makes function dispatch slightly more efficient in CPU
usage, and furthermore reduces stack usage for these cases.  On x86 and
Thumb archs the dispatch functions are now tail-call optimised by the
compiler.

The bare-arm port has its code size increase by 76 bytes, but stmhal drops
by 904 bytes.  Stack usage by these builtin functions is decreased by 48
bytes on Thumb2 archs.

The timeval struct can be defined by system C headers.