If step is not constant, in first approximation, we can't apply optimization,
(well, we could, but need a special case for this).

Creating of classes (types) and instances is much more like CPython now.
You can use "type('name', (), {...})" to create classes.

Use computed string to exercise string interning code better.

Added 0 to the list of primes. Funky primes, these.

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.

list.index fails on an edge case.

Fixes issue #63.