Separate serde serialization from base code, so that it can be skipped using Cargo features

[krial057]

It would be nice to be able to compile the misp_types crate without serialization features/serde. There is already a json-using-serde feature configured in the Cargo.toml. However, I make use of alot of serde derives which makes it difficult to split the code depending on the feature configurations. Would be nice to create manual serde serializations/deserializations functions instead of using the derives. This way, the serde dependency can easily be added/removed depending on the feature set.