Implement printing in the C++ emulator instead of in Verilog

[lsk567]

The printing logic is currently implemented in flexpret/scripts/hdl/simify_verilog.py which monitors the tohost register in Verilog. As we aim to support more advanced printing (such as printf), it would make sense to move the printing logic to the emulator instead of putting everything in flexpret/scripts/hdl/simify_verilog.py, which should ideally stay small.