This patch allows the following code to run without allocating on the heap:

    super().foo(...)

Before this patch such a call would allocate a super object on the heap and
then load the foo method and call it right away.  The super object is only
needed to perform the lookup of the method and not needed after that.  This
patch makes an optimisation to allocate the super object on the C stack and
discard it right after use.

Changes in code size due to this patch are:

   bare-arm: +128
    minimal: +232
   unix x64: +416
unix nanbox: +364
     stmhal: +184
    esp8266: +340
     cc3200: +128

This patch refactors the handling of the special super() call within the
compiler.  It removes the need for a global (to the compiler) state variable
which keeps track of whether the subject of an expression is super.  The
handling of super() is now done entirely within one function, which makes
the compiler a bit cleaner and allows to easily add more optimisations to
super calls.

Changes to the code size are:

   bare-arm: +12
    minimal:  +0
   unix x64: +48
unix nanbox: -16
     stmhal:  +4
     cc3200:  +0
    esp8266: -56

If we get to this point in the code then pn_iter is guaranteed to be a
struct.

Prior to making this a config option it was previously available on these
(and all other) ports, and it makes sense to keep it enabled for mpy-cross
as well as ports that have a decent amount of space for the code.

With this optimisation enabled the compiler optimises the if-else
expression within a return statement.  The optimisation reduces bytecode
size by 2 bytes for each use of such a return-if-else statement.  Since
such a statement is not often used, and costs bytes for the code, the
feature is disabled by default.

For example the following code:

    def f(x):
        return 1 if x else 2

compiles to this bytecode with the optimisation disabled (left column is
bytecode offset in bytes):

    00 LOAD_FAST 0
    01 POP_JUMP_IF_FALSE 8
    04 LOAD_CONST_SMALL_INT 1
    05 JUMP 9
    08 LOAD_CONST_SMALL_INT 2
    09 RETURN_VALUE

and to this bytecode with the optimisation enabled:

    00 LOAD_FAST 0
    01 POP_JUMP_IF_FALSE 6
    04 LOAD_CONST_SMALL_INT 1
    05 RETURN_VALUE
    06 LOAD_CONST_SMALL_INT 2
    07 RETURN_VALUE

So the JUMP to RETURN_VALUE is optimised and replaced by RETURN_VALUE,
saving 2 bytes and making the code a bit faster.

Otherwise the type of parse-node and its kind has to be re-extracted
multiple times.  This optimisation reduces code size by a bit (16 bytes on
bare-arm).

PN_atom_expr_normal parse nodes always have structs for their second
sub-node, so simplify the check for the sub-node kind to save code size.

Saves code size (20 bytes on bare-arm) and makes it a tiny bit more
efficient.

Add definitions/source files for features which work on the windows
ports but weren't yet enabled.
UTIME related lines are moved a couple of lines up to make comparision
with unix/mpconfigport.h easier in the future.

With 30-bit floats there aren't enough bits to faithfully print 7 decimal
digits, so reduce the precision to 6 digits.

Sometimes when setting a channel callback the callback fires immediately,
even if the compare register is set to a value far into the future. This
happens when the free running counter has previously been equal to what
happens to be in the compare register.

This patch make sure that there is no pending interrupt when setting a
callback.

It controls the character that's used to (asynchronously) raise a
KeyboardInterrupt exception.  Passing "-1" allows to disable the
interception of the interrupt character (as long as a port allows such a
behaviour).

cc3200 has been updated to conform to the API and now returns None.

This aligns the I2C class to match the standard machine.I2C API.

Note that this is a (small) breaking change to the existing cc3200 API.
The original API just returned the size of the input buffer so there's no
information lost by this change.  To update scripts users should just use
the size of the buffer passed to these functions to get the number of bytes
that are read/written.

The cc3200 port is now similar enough to the standard machine.I2C API so
that all conditionals can be removed.

This is a user-facing change to the cc3200's API, to make it conform to the
new machine hardware API.  The changes are:

- change I2C constructor to: I2C(id=0, *, freq=100000, scl=None, sda=None)
- change I2C init to: init(*, freq, scl, sda)
- removal of machine.I2C.MASTER constant
- I2C str/repr no longer prints I2C.MASTER

To update existing code it should be enough to just remove the I2C.MASTER
constant from contructor/init for I2C.

stmhal doesn't have a machine.UART class so this section is not needed.

If we got a CRASH result, return early, similar to SKIP. This is important
because previous refactor changed branching logic a bit, so CRASH now gets
post-processed into CRASH\n, which broke remote hardware tests.

Don't give a guarantee of HW RNG, only a possibility of its usage.

A shorter name takes less code size, less room in scripts and is faster to
type at the REPL.

Tests and HW-API examples are updated to reflect the change.

As Zephyr currently doesn't handle MTU itself (ZEP-1998), limit amount
of data we send on our side.

Also, if we get unsuccessful result from net_nbuf_append(), calculate
how much data it has added still. This works around ZEP-1984.

If run-tests script is run from another dir, we still want to look up
feature checks in run-tests' dir.

Give a clearly dissuading tone on end users trying 512KB version
- it has to many end-usery features lacking.

Instead of complicating recv() implementation.