Serialize enums as maps by default

[pyfisch]

This matches other serde formats.
Directly set options on Serializer.
Remove rarely used to_* methods.

Closes #85

[pyfisch]

Search results reveal that to_vec, to_writer and to_vec_packed are primarily used. Other methods such as to_vec_sd are only rarely needed. Therefore users should directly call the Serializer in this case.

[pyfisch]

Regarding #117, I felt that a config struct without accessors may hinder future extensibility and a separate builder was too much complexity.

It is now possible in theory so change the options in the middle of serialization and shoot yourself in the foot but this requires some effort on the users side.

[sfackler]

Yeah, I think just having setters on Serializer/Deserializer for options makes sense.