Make some things constexpr

[goto-bus-stop]

This constexpr-ifies just enough things to allow for a constexpr Guid(std::string_view) constructor. Now you can write readable GUIDs at compile time:

static constexpr auto SomeGuid = xg::Guid("AF465C71-9588-11cf-A020-00AA006157AC");

After that, there was barely any code left in the guid.cpp file. I moved the remaining short functions into the header file too. Only newGuid() requires that users add the guid.cpp file to their compilation, to avoid pulling things like the windows.h header or a libuuid dependency in the header file. This is kinda just a thing I like but not necessary by any stretch so if you don't prefer it I'm happy to revert that.

Some of the constexpr methods have manually written loops instead of non-constexpr std::fill and stuff. isValid() and zeroify() are both required by the Guid(std::string_view) constructor so I had to make do to make that constexpr :)

Also updated CI to work again. The MATRIX_EVAL stuff appears to be broken despite Travis recommending it in their docs? Weird. Removed the Homebrew update that was taking ages, Travis must've updated the CMake version on their OSX image at some point because it's now recent enough.