piranha.hpp: todo item.
mp_rational: multiplication.
mp_rational: implement math::is_zero().
mp_rational: division.
polynomial: fix a problem with the deduction of the result type of a functor
in some systems (SuSE and Fedora, apparently).
series_multiplier: ditch the deprecated random_shuffle in favour of shuffle.
polynomial: replace sorts with stable sorts, to enforce consistency
of performance across platforms.
Switch back to integer in the gastineau benchmarks for the moment.
build system: change the naming convention for the source files
of perf tests, so we can have a debug and a release test of the same
functionality.

Also, fix some missing headers in the memory test.
memory: in perf test, allow to specify the max number of threads to use.
mp_integer: add support for ignoring upper bits in the return value for read_uint().

This is a prerequisite for future performance improvements.
Merge branch 'master' into quadmath