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]

Thanks for the PR. This looks pretty good. I have a couple of small problems

First: Can you please make some kind of test that demonstrates the use of the flag and that
the .fst file was created. GcdExampleSpec, for instance, test whether a .vcd was created.

Second: There are quite a few changed lines in this PR that appear to be cosmetic, different spacing.
It makes it harder for the reviewer to focus on the core change.
If you'd like to reformat things to be better style compliant that's great, but best to keep that in a separate style PR.

[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.