On stm32 boards, machine.I2C is now preferred over pyb.I2C.

This allows to efficiently send to an I2C slave data that is made up of
more than one buffer.  Instead of needing to allocate temporary memory to
combine buffers together this new method allows to pass in a tuple or list
of buffers.  The name is based on the POSIX function writev() which has
similar intentions and signature.

The reasons for taking this approach (compared to having an interface with
separate start/write/stop methods) are:
- It's a backwards compatible extension.
- It's convenient for the user.
- It's efficient because there is only one Python call, then the C code can
  do everything in one go.
- It's efficient on the I2C bus because the implementation can do
  everything in one go without pauses between blocks of bytes.
- It should be possible to implement this extension in all ports, for
  hardware and software I2C.

Further discussion is found in issue #3482, PR #4020 and PR #4763.

It's more common to need non-blocking behaviour when reading from a UART,
rather than having a large timeout like 1000ms (the original behaviour).
With a large timeout it's 1) likely that the function will read forever if
characters keep trickling it; or 2) the function will unnecessarily wait
when characters come sporadically, eg at a REPL prompt.

Selectable at compile time via MICROPY_PY_UCRYPTOLIB_CTR.  Disabled by
default.

MODULE_EXAMPLE_ENABLED must be globally defined for the module to be seen
and referenced by all parts of the code.

As already mentioned in the docs, not all constants may be available on all
ports, so this is optional to implement.

How to use this feature is documented in docs/develop/cmodules.rst.

This system makes it a lot easier to include external libraries as static,
native modules in MicroPython.  Simply pass USER_C_MODULES (like
FROZEN_MPY_DIR) as a make parameter.

This is only correct for the extmod/uos_dupterm.c implementation however,
as e.g cc3200 implementation does the mp_load_method() itself, and anyway
requires `read` instead of `readinto`.

Replaces "PYB: soft reboot" with "MPY: soft reboot", etc.

Having a consistent prefix across ports reduces the difference between
ports, which is a general goal.  And this change won't break pyboard.py
because that tool only looks for "soft reboot".

The machine.sleep() function can be misleading because it clashes with
time.sleep() which has quite different semantics.  So change it to
machine.lightsleep() which shows that it is closer in behaviour to
machine.deepsleep().

Also, add an optional argument to these two sleep functions to specify a
maximum time to sleep for.  This is a common operation and underlying
hardware usually has a special way of performing this operation.

The existing machine.sleep() function will remain for backwards
compatibility purposes, and it can simply be an alias for
machine.lightsleep() without arguments.  The behaviour will be the same.

With contributions from Oliver Robson (@HowManyOliversAreThere), Sean
Lanigan (@seanlano) and @rprr.

The docs are now built as one for all ports.

To get rid of warning when building the docs saying there's a redirect from
http: to https:.

Examples are added to the beginning of the module docs, similarly to docs
for many other modules.

Improvements to grammar, style, and clarity. Some paragraphs are updated
with better suggestions. A warning added of the effect incorrect usage of
the module may have. Describe the fact that offset range used in one
defined structure is limited.

Otherwise there is really nothing that can be done, it can't be unlocked by
the user because there is no way to allocate memory to execute the unlock.

See issue #4205 and #4209.

E.g., register() can be called again for the same object, while modify()
will raise exception if object was not register()ed before.