Supports suspend and hibernate modes. Waking is possible throug GPIO
and WLAN.
The mpcallback class is generic and can be reused by other classes.

Also add another method to change the pin's interrupt mode
on the fly.

Such functions are never used after MicroPython has started, and they
remain in RAM wasting space. Now they are placed in a special section
named "boot" which sits just before the heap, allowing us to extend
the effective heap area up to the new boot section. Right now, this
gives us back ~1K, but in the future, more functions might end up in
there as well.

This follows existing pattern for object constructor API and allows to
create memoryview objects e.g. in external modules.

Might be useful at least for memoryview hacks.

This helps make files reusable across other ports.

See issue #1122.

Just to reduce code size.  Messages are still to the point and
unambiguous.