actix-web-location: Use compile_error! to generate feature-related errors

[ethowitz]

We are currently using deprecation warnings to generate compiler messages when either both or neither of the actix-web-v3 and actix-web-v4 features are being compiled. Instead, let's use the compile_error! macro to generate compiler errors:

#[cfg(all(feature = "actix-web-v3", feature = "actix-web-v4"))]
compile_error!("Only one of actix-web-v3 and actix-web-v4 can be used at once.");

#[cfg(not(any(feature = "actix-web-v3", feature = "actix-web-v4")))]
compile_error!("Exactly one of actix-web-v3 or actix-web-v4 must be specified.");