Add casting from Rational to a floating-point type.
BREAKING CHANGE.

Reorganize the source tree to avoid the import path mess: https://github.com/rejectedsoftware/dub/issues/197

This fixes many dub builds (like Mono-D, VisualD not-combined) that I would have broken in commit https://github.com/p0nce/gfm/commit/3f1c940d1ce7baf74b507819c39faa15d9230ce8.

Unrelated: there is currently another problem when building with dub; you have to use the --combined switch since separate compilation of gfm:sdl2 fail for some reason.