Fix behaviour of simd::allocator with types with overloaded new

boost::simd::allocator<T>::construct was calling the placement
new operator incorrectly, leading to problem when the allocator
was dealign with types with overloaded new operator.
Adde dmissing placement-new overload in boost.simd
Fixed type mistake in Twotonmb unit test
Remove improper type usage in benchs. Part of issue #256
Fixed hermite include
Write ad hoc bench for polynomials functions
Make nary_benchmark works with arbitrary output types

This fixes 90% of issue #256 by making all multi-return
function bench compiling and running.