Speedup container conversion

Boost::python extract is actually quite slow, as it performs a lot of
checks. Let's be wild and speedup the process!

Note that we still use Boost::python::extract once just to spot easy
user mistakes.
Add an extra test case

Works like a charm!
Improve slicing by specializing for step = 1

When step is one, many formulae no longer require a multiply. This
improves a lot the performance of several kernels, for a relatively low
cost.

As a side effect, renamed list_view into sliced_list and str_view into
slcied_str, using the same naming scheme as in ndarray.
Do not make useless copies in tuple constructor
Rework handling of Python comparison

In case of multiple comparisons, previous implementations evaluated all
arguments before doing the comparison. This is a bad behavior, as
evaluation of boolean expression are done lazily.

This improves the performance of the nqueens benchmarks a lot!
Remove useless const_cast in sliced_str constructor

That was some really ugly legacy code! Thanks Joker-Eph for spotting it!
Remove useless non const-qualified operators

Again, Joker-eph is the one to praise there.