Add `operator""` for doubles

[vabold]

The current implementation we have uses 0.3d, which is fine for gcc but breaks clang. operator"" will allow for custom suffixes to be used.

[malleoz]

Where will YOU stand in the Great Suffix War of Kinoko?

I'm gonna be honest, adding overhead in order to support an inconsistently supported suffix seems like something we shouldn't bother doing. I understand your readability argument, even if m8 doesn't agree necessarily. But then again, we don't use doubles very often. Once precision-related desyncs are fixed, we don't necessarily care about readability because the hope is that we don't ever have to worry about the precision there again.

[adrien-obrecht]

Why not just static_cast<double>(0.3) ? The _d solution seems so convoluted for just one (or very few) instance