Thanks to @robert-hh.

All arguments to pin.irq are converted from keyword-only to positional, and
can still be specified by keyword so it's a backwards compatible change.

The default value for the "trigger" arg is changed from 0 (no trigger)
to rising+falling edge.

For polymorphic interfacing on C level.

Otherwise, they serve reoccurring source of copy-paste mistakes and
breaking nanbox build.

Previously they used historical "pyb" affix causing confusion and
inconsistency (there's no "pyb" module in modern ports; but people
took esp8266 port as an example, and "pyb" naming kept proliferating,
while other people complained that source structure is not clear).

Pin(16) now works as an input.

They almost certainly needed by any C code which calls Python callbacks.

Most pin I/O can be done just knowing the pin number as a simple
integer, and it's more efficient this way (code size, speed) because it
doesn't require a memory lookup to get the pin id from the pin object.

If the full pin object is needed then it can be easily looked up in the
pin table.

Now pins can be easily looked up in the table using the pin number as the
index and vice versa.

This field is the same as phys_port and not needed.

This is an initial attempt at making a simple C pin API for writing
things like I2C drivers in C.

GPIO16 is actually special-function I/O, though some boards have LED there.

OPEN_DRAIN is of course synthesised.  All pin modes are tested and
working.

The first argument to the type.make_new method is naturally a uPy type,
and all uses of this argument cast it directly to a pointer to a type
structure.  So it makes sense to just have it a pointer to a type from
the very beginning (and a const pointer at that).  This patch makes
such a change, and removes all unnecessary casting to/from mp_obj_t.

1. Updated linker script, now user app appears to contain exception vector
table and oesn't work (faults) without it.
2. Commened out support for GPIO pulldown, which was removed in this SDK
version without clear explanation, but apparently because it was released
without proper validation, and now turns out it doesn't work as expected,
or there's a different function there.

Previous to this patch the printing mechanism was a bit of a tangled
mess.  This patch attempts to consolidate printing into one interface.

All (non-debug) printing now uses the mp_print* family of functions,
mainly mp_printf.  All these functions take an mp_print_t structure as
their first argument, and this structure defines the printing backend
through the "print_strn" function of said structure.

Printing from the uPy core can reach the platform-defined print code via
two paths: either through mp_sys_stdout_obj (defined pert port) in
conjunction with mp_stream_write; or through the mp_plat_print structure
which uses the MP_PLAT_PRINT_STRN macro to define how string are printed
on the platform.  The former is only used when MICROPY_PY_IO is defined.

With this new scheme printing is generally more efficient (less layers
to go through, less arguments to pass), and, given an mp_print_t*
structure, one can call mp_print_str for efficiency instead of
mp_printf("%s", ...).  Code size is also reduced by around 200 bytes on
Thumb2 archs.