Implement F (or Zfinx)

[tilk]

We need to implement floating point operations in order to support interesting applications. It probably isn't worth it to implement IEEE 754 in Amaranth, so the best course of action is to use an existing module (probably written in Verilog).

It should be checked how well Amaranth cooperates with external Verilog modules (Instance objects). If external Verilog is not simulated in pysim, the floating point operations should be implemented in Python, and a stub used to integrate with the simulation.