Partly addresses issue #856.

If micropython.native decorator doesn't compile, then we skill all
native/viper tests.

This patch also re-enables the ujson_loads test on NT.

Addresses issue #861, and partially addresses issue #856.

Found these by compiling stmhal with mp_uint_t of type uint32_t instead
of unsigned int.  This actually makes a difference to the code, but just
a curiosity.

This has benefits all round: code factoring for parse/compile/execute,
proper context save/restore for exec, allow to sepcify globals/locals
for eval, and reduced ROM usage by >100 bytes on stmhal and unix.

Also, the call to mp_parse_compile_execute is tail call optimised for
the import code, so it doesn't increase stack memory usage.

Timers now have the following new features:
- can init freq using floating point; eg tim.init(freq=0.1)
- tim.source_freq() added to get freq of timer clock source
- tim.freq() added to get/set freq
- print(tim) now prints freq

windows tests fixes

Eg pyb.freq(120000000) sets the CPU to 120MHz.  The frequency can be set
at any point in the code, and can be changed as many times as you like.
Note that any active timers will need to be reconfigured after a freq
change.

Valid range is 24MHz to 168MHz (but not all freqs are supported).  The
code maintains a 48MHz clock for the USB at all times and it's possible
to change the frequency at a USB REPL and keep the REPL alive (well,
most of the time it stays, sometimes it resets the USB for some reason).
Note that USB does not work with pyb.freq of 24MHz.

Implement missing ARM emitter functions for viper

This should pretty much resolve issue #50.

Addressing issue #50.

Comes with test script.  Copy both files to pyboard and run
"import nrf24l01test".

Remove reference to pyb.gc; add reference to pyb.millis.

There are lots of functions not listed when you run help(), but it would
be too much to list them all, so we list only some basic, useful ones.

Addresses issue #846.

Addresses issue #859.

3rd party drivers should not export generic names like "close".

As per issue #876, the network module is used to configure NICs
(hardware modules) and configure routing.  The usocket module is
supposed to implement the normal Python socket module and selects the
underlying NIC using routing logic.

Right now the routing logic is brain dead: first-initialised,
first-used.  And the routing table is just a list of registered NICs.

cc3k and wiznet5k work, but not at the same time due to C name clashes
(to be fixed).

Note that the usocket module has alias socket, so that one can import
socket and it works as normal.  But you can also override socket with
your own module, using usocket at the backend.

Most of the time you don't use the NSS pin of the SPI bus, and so it
shouldn't be enabled by default (this gave some bugs in the past).

In CPython IOError (and EnvironmentError) is deprecated and aliased to
OSError.  All modules that used to raise IOError now raise OSError (or a
derived exception).

In Micro Python we never used IOError (except 1 place, incorrectly) and
so don't need to keep it.

See http://legacy.python.org/dev/peps/pep-3151/ for background.