Uniformity in the use of Cow instead of String

[nappa85]

Cow is used in all deserializable structs instead of String to gain uniformity lookup_value takes a mutable reference and replaces in place instead of returning the new value Test code simplified using Into instead of From for better readability

[lu-zero]

beside the typo in the doctest ^^

[allevo]

Why the code force to have 'static lifetime? Is it really needed?

[nappa85]

Why the code force to have 'static lifetime? Is it really needed?

Because this way we don't have explicit lifetimes in all structs, serde deserializes in an owned String anyway and lookup_value already uses static strings. Using a named lifetime seems like an overcomplication to me