meta/Expr.h: Less boilerplate, fix return type of fmin/fmax.
meta/Expr.h: Cosmetic changes.
PrinterMain: Rename Cache to Config.
Expr, Configuration: Make constexpr work properly.

We need a macro for retrieving from ConfigCache if constexpr is to be propagated.
Expr: Implement FP constant definitions in a nicer way.

It's not nice to define value() in the anonymous struct because Expr defines value() as well.