Implement VCD waveform file dump

[mortbopet]

VCD waveforms are a common tool for analyzing simulations of digital circuits.

The implementation should be on a vsrtl::widget level, wherein it should connect to the Gallant::Signal members available in components and ports. These signals are emitted when ports and components change value - exactly what is required for the VCD file format.

Writing a VCD writer should be (fairly) simple - however, the file format should be used to its advantage, wherein all signals and components are nested within their respective parents, as the file format allows.

Resources:
https://en.wikipedia.org/wiki/Value_change_dump
https://github.com/wohali/vcd_parsealyze

#7 should be resolved before this.