Switch from deku to cookie-factory and nom

[aumetra]

deku is a bit overkill for a small parser like this. We don't have to perform any bit level manipulation nor is the format itself complex enough to warrant using a parser generator

The switch also gets rid of proc macros and the alloc requirement (making this crate entirely no_std compatible)

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I'm not entirely sure whether the deserialise and serialise functions are idiomatic to cookie-factory/nom (as in that they can be used in generator/parser chains)