Let map types be serialized directly

[ElnuDev]

Currently, the MapType trait cannot be serialized directly, since serde doesn't support serialization of traits (i.e. Box<dyn MapType>). As a work around, I'm using the Map enum with a variant for each individual map type. This works, but it's extremely verbose and requires a lot of duplicated code for each new map type.

Apparently, the typetag crate will fix this problem—it adds the ability to serialize and deserialize trait objects to serde.