Module _scons_itertools

Implementations of itertools functions for Python versions that don't
have iterators.

These implement the functions by creating the entire list, not returning
it element-by-element as the real itertools functions do.  This means
that early Python versions won't get the performance benefit of using
the itertools, but we can still use them so the later Python versions
do get the advantages of using iterators.

Because we return the entire list, we intentionally do not implement the
itertools functions that "return" infinitely-long lists: the count(),
cycle() and repeat() functions.  Other functions below have remained
unimplemented simply because they aren't being used (yet) and it wasn't
obvious how to do it.  Or, conversely, we only implemented those functions
that *were* easy to implement (mostly because the Python documentation
contained examples of equivalent code).

Note that these do not have independent unit tests, so it's possible
that there are bugs.