Consider merging all error types into sfv::Error

[apasel422]

It's unclear if having separate DecimalError, KeyError, TokenError, StringError, and OutOfRangeError types is actually useful. All of these correspond to cases in which data is being converted into an sfv type, the majority of which will occur during serialization, which is already fallible for lists and dictionaries, which return sfv::Error to indicate this.

Having a common error type (i.e. using sfv::Error for all of these) makes it a bit easier to propagate them without falling back to Box<dyn std::error::Error> or a user-defined enum.

See https://docs.rs/jiff/latest/jiff/struct.Error.html#design for related discussion of this pattern.