The aim here is to improve coverage of the code.

See issue #1122.

So that navite emitter passes (comprehensions use yield which is not yet
supported by native emitter).

This makes exception traceback info self contained (ie doesn't rely on
list object, which was a bit of a hack), reduces code size, and reduces
RAM footprint of exception by eliminating the list object.

Addresses part of issue #1126.

Issue #1122 should now be fixed.

Addresses issue #1117.

Addresses issue #1109.

Previous to this patch, a big-int, float or imag constant was interned
(made into a qstr) and then parsed at runtime to create an object each
time it was needed.  This is wasteful in RAM and not efficient.  Now,
these constants are parsed straight away in the parser and turned into
objects.  This allows constants with large numbers of digits (so
addresses issue #1103) and takes us a step closer to #722.

Addresses issue #1097.

Also fix list.sort so it works with user-defined types, and parse the
keyword arguments properly.

Addresses issue #338.

Only noticeable difference is how newlines are encoded in triple-quoted
strings.  The behaviour now matches CPython3.