VS2013 compile fail in cpprest/streams.h

[garethsb]

Commit 05e474a causes compile to fail in VS2013, for example at cpprest/streams.h(895):

error C2899: typename cannot be used outside a template declaration

That's probably a bug in VS2013, but I'm pretty confident all the typenames and typedefs used in this file can be hugely simplified while regaining VS2013 compatibility. I'll submit a PR.