Release notes for Hana 1.6.1

Official support for Xcode 6, 7 and 8, 9, 10 and LLVM Clang 3.5, 3.6, 3.7, and 3.8 has has been dropped. The library should still work with these compilers, however they are not being tested regularly anymore, so they are not officially supported.
The hana::traits::result_of trait has been removed. Since std::result_of has been removed from the Standard in C++20, users should move away from it.