Add bound attribute to restrict derived impls

[MegaBluejay]

Analogous to serde's

This allows using generics without restricting them in the struct definition

The use-case for me is parsing into a raw form, then post-processing some fields, while retaining most of the structure. With this both can use the same generic type, and only the raw form needs to implement DekuRead/DekuWrite.

Inferring the bounds automatically would be better, but I'm not up to writing that right now.

[wcampbell0x2a]

Could you add documentation to src/attributes.rs? Thanks