mp_obj_module_get_globals() returns a mp_obj_dict_t*, and type->locals_dict
is a mp_obj_dict_t*, so access the map entry of the dict directly instead
of needing to cast this mp_obj_dict_t* up to an object and then calling the
mp_obj_dict_get_map() helper function.

This patch simplifies the str creation API to favour the common case of
creating a str object that is not forced to be interned.  To force
interning of a new str the new mp_obj_new_str_via_qstr function is added,
and should only be used if warranted.

Apart from simplifying the mp_obj_new_str function (and making it have the
same signature as mp_obj_new_bytes), this patch also reduces code size by a
bit (-16 bytes for bare-arm and roughly -40 bytes on the bare-metal archs).

As a pointer (const char *) it takes up an extra word of storage which is
in RAM.

Reduces code size by about 10 bytes.

This is how CPython does it, and it's very useful to help users discover
the available modules for a given port, especially built-in and frozen
modules.  The function does not list modules that are in the filesystem
because this would require a fair bit of work to do correctly, and is very
port specific (depending on the filesystem).

This builtin is configured using MICROPY_PY_BUILTINS_HELP, and is disabled
by default.

This patch introduces MP_PYTHON_PRINTER for general use.

Several ports use identical code for the 1-argument form of the builtin
help function.  Move this code to a library function to allow easier
re-use by ports.