[stringhandler]

There is a weird non-serde feature set in simc. This makes writing tests that read and write data difficult.

I can't see a place where we would be worried about having serde in the tests, but if there is one, I'm happy to close this pr.

LLM description below:

Tests that read witness or argument files needed a cfg(feature = "serde") gate, so cargo test --no-default-features skipped 52 of them. Make the crate a dev-dependency of itself with serde enabled, so every test build has serde, and drop serde from the 30 test-only cfg attributes.

The library and simc still build without serde; CI checks that with cargo build --no-default-features and cargo hack.

Assisted-by: Claude

Checklist

• If this PR adds or changes a language feature, it is used in
  examples/kitchen_sink.simf, and test-data/kitchen_sink.json is
  regenerated.

[apoelstra]

Personally I think we should just make serde non-optional. We use it for pretty basic functionality. To the extent this is a problem for anybody, we should fix it by splitting up the library into multiple crates, some of which maybe could be serde-free.