Move FilesystemExporter from trait object to generic

[Manishearth]

I don't know why we wanted to do this, but it's listed in #2856 so I did it. Feel free to close if we don't want to do it anymore.

[sffc]

Did some archaeology and it seems I added this to the checklist on September 12, 2023. I don't see any PRs that landed within a week of that date that would have caused me to add this.

I think what I was probably thinking was:

1. The serializers module has always been a bit clunky and it would be nice if AbstractSerializer weren't public
2. We only really support a small number of formats: JSON, Postcard v1, and Bincode v1. We haven't tested it with other formats, and Serde doesn't guarantee that it will work with other formats.
3. We don't like trait objects, so this is clearly an opportunity to move away from it
4. So, maybe we should move away from the trait and instead just have these be options. In other words, change this from trait object to enum, not a generic.

I don't think I would have considered a generic to be a better situation than the trait object, because the generic probably increases code size in this situation, since the generic function is deep, not shallow.

[sffc]

I suggest we make this a 2.0-stretch issue and discuss it there.