Support for FST traces in verilator backend

[kammoh]

Introducing the ability to use FST format as the output format of Verilator trace dump, instead of the VCD format. FST is a more efficient trace format introduced by the gtkwave project. A boolean option named verilatorFstTrace is currently added to TesterOptions, but can be eventually ported into chisel-testers2 with an appropriate Annotation.

[chick]

@ekiwi I know you have some interest in .fst format, or is your use case just for treadle

[ekiwi]

Thanks for adding me @chick. This looks like a great pull request (once the formatting issues are addressed).

Do you know if the verilator version is available in the backend? As far as I remember the fst support was only added in verilator 4.0, thus it might be good to catch any versions below that and fail with a useful error message to the user, telling them to update their verilator version if they need fst output.