Fixes issue #63.

Only step=1 and non-negative indexes are supported so far.

So far, only start and stop integer indexes are supported. Step is
not supported, as well as objects of arbitrary types.

This in particular makes it available for stm port.

* Fixes issue #51
* Adds a specific error message for when you try to pop an empty list.
* Releases some memory if the list has shurnk a lot.

mpconfig.h will automatically pull mpconfigport.h.

This is just max value of current_bytes_allocated seen.

Unlike total_bytes_allocated, this tracks m_free()'s too.

import works for simple cases.  Still work to do on finding the right
script, and setting globals/locals correctly when running an imported
function.

Generator objects now allocate the object and the state in one malloc.
This improvement fixes Issue #38.

This is based on the fact that qstr so far behaves more like byte string
than like Unicode string (for example, "012"[0] returns 48 (int)).

This fixes Issue #29, and means the core is no longer dependent on
string functions, except strlen.