Add a `de` feature to complement `ser`

[BrandonDyer64]

I can serialize the Entropy struct, but have no way to deserialize it without making my own struct.

I propose adding

#[cfg_attr(feature = "de", derive(Deserialize))]

[shssoichiro]

It probably makes sense to just bake it into the existing ser feature. The reason the feature would be desired is to avoid depending on serde if not needed. If we enable the feature we may as well use it for both Serialize and Deserialize.

[Emm]

@shssoichiro, the pull request above adds Deserialize support with the ser flag. Please note that it introduces non-backward compatible changes since it changes &'static str fields in favor of String fields.