Use strum to replace manual implementation of as_str and FromStr

[crazytonyli]

Here are the two core changes:

1. Use strum_macros::EnumString derive macro to replace our own FromStr implementation.
2. Use strum_macros::Display derive macro to replace our own as_str implementation. The macro adds std::fmt::Display implementation to the attached enums.

Breaking change

The enums now no longer have the as_str function. Instead, they have to_string.

Do not use strum_macros::AsRefStr

strum also has a derive macro AsRefStr to add fn as_ref() -> AsRef<str> to the attached enums. However, the function does not match the to_string implementation generated by strum_macros::Display macro. The Custom(String) variant returns "Custom", instead of the String value in the tuple.

[oguzkocer]

@crazytonyli I left one important comment about #[strum(serialize = "{value}")], but otherwise this looks good to me.