The Constant wrapper can hold any Synth type
and store it in a circuit for use by the HDL kernel.
This is the easiest way to compute complex constants
in your RustHDL constructors, and then store the
results inside the circuit for later use. Unlike
Signal, Constant does not have a .next field,
so you cannot assign to a Constant in the HDL
kernel (blocked at compile time). Note that Constant
does not impl Default. You must construct it
with the appropriate value when the circuit is built.