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Addresses Issue #203.

This reuses as much str implementation as possible, from this we
can make them more separate as needed.

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

Note that splitting by explicit string is not implemented so far.

Also, support negative indexes.

That at least makes stuff like "{:x}".format(1) to produce not completely
broken output.

It's not really about that, though; it's about me figuring out a sane
way forward for keyword-argument functions (and function
metadata). But it's useful as is, and shouldn't break any existing
code, so here you have it; I'm going to park it in my mind for a bit
while sorting out the rest of the dict branch.

Now much more inline with how CPython does types.

Qstr's are now split into a linked-list of qstr pools.  This has 2
benefits: the first pool can be in ROM (huge benefit, since we no longer
use RAM for the core qstrs), and subsequent pools use m_new for the next
pool instead of m_renew (thus avoiding a huge single table for all the
qstrs).

Still would be better to use a hash table, but this scheme takes us part
of the way (eventually convert the pools to hash tables).

Also fixed bug with import.

Also improved the way the module code is referenced (not magic number 1
anymore).

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

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