Use external half hpp

[airlied]

This imports half.hpp from half.sourceforge.net which is a MIT licenses half float implementation.

No idea if this is the best way to do this, but I reckon it's a good start. I've had to modify the API one bit to match the conformance tests.

(this is based on my vector swizzle branch in #195)

[keryell]

What is the current status of this? We are just discussing something similar in https://github.com/intel/llvm/pull/78#discussion_r274738232