Partial loading of skf

[johnlees]

If the skf is very large, loading and decompressing the whole thing is slow. It might be possible just to define and get a header struct, which would be useful for ska nk.

[johnlees]

Could try #[serde(skip_deserializing)] on variants fields

[johnlees]

Tried skip_deserialising trait, and making a smaller struct which omits the large fields. Both take just as long to read in. It's possible this is due to either the CBOR format used (I think more likely) or the compression, but either way this would require a breaking change to the .skf format to work. I've got a couple of thoughts about that, but I will track them in #17 instead.