Fixed dlhn benchmark to work properly

[otake84]

Thank you for choosing dlhn as a benchmark target. I have made the following changes to make the dlhn benchmark work:

1. Changed the type of the LargeStruct map field from HashMap<u32, u64>, to HashMap<String, u64>. This is because dlhn can only use strings as Map types.
2. In Rust, floating-point numbers cannot be used as keys for HashMap or BTreeMap, so we made a bold decision to design dlhn to only allow strings as keys for Maps.
3. Currently, dlhn does not support 128-bit integers, so I have made changes to disable the dlhn benchmark when the model_128 option is enabled.
4. Lastly, I fixed the code to work by uncommenting benches!(serde_cbor);, which was commented out along with the fixme comment.

I hope that these changes will help improve the benchmarking process for musli. Thank you for your support and understanding.

[udoprog]

Seems reasonable to me. Thanks for the rundown!