Hash: Remove C++11 forward declarations.
It seems that on Visual C++ Dinkumware declares shared_ptr in std, but for
Intel C++ it imports it from std::tr1. I was thinking of doing a macro check
for this, but since we're close to release, just change it to always include
the headers and never forward declare the C++11 classes, which is much safer.
I might restore the forward declarations in the future, although I'm tempted
not to. I'm not sure the improvement in compile time is worth the hassle.