Migrate source/build to c++14 ...

Migrate source/build to c++14                                                                                                                    (#522)

* Update `CMakeLists.txt` for C++14 option.
* Update to gcc 6 minimum.
* Update travis CI from gcc-5 to gcc-6
* Use `std::..._t` style type traits, replacing `util::` aliases.
* Use `std::cbegin`, `std::cend`, and `std::make_unique`, replacing `util::` versions.
* Remove `DEDUCED_RETURN_TYPE` macros.
* Remove redundant return type specifications.
* Use correct ADL for `begin` and `end` in (almost all) the range utilities.
* Remove redundant `mechinfo` ctor (aggregate initialization suffices).
* Use lambda capture initializers where appropriate.
* Use generic `std::equal_to`.
* Use variable templates for `math::infinity` and `math::pi`.
* Remove `enum_hash` workaround.
* Use `""s` string literals where we were using our own `""_s` construction.
* Use generic lambda for recursive lambda instead of `std::function` wrapper.
* Use generic lambda for generic arithmetic tests.

Fixes #358.