Split overloaded if construct into if and when for compiletime and runtime conditions

[VonTum]

We should use an extra keyword when to distinguish runtime conditions from compiletime codegen conditions?

There's a semantic difference between them. if prevents a section of hardware from being generated, while when just adds runtime conditions to it.

The big benefits are clarity of intent on the programmer's part. Also the benefit is the compiler can warn the programmer when using a when with a compile time constant, or an error on if with a non-constant.

The rest of the syntax can be entirely shared!