Reduce size of Error enum

[frewsxcv]

At the time of writing, the Error enum is 216 bytes. We can reduce the size by splitting the enum into different enums (e.g. DeserializeError, SerializeError, ToGeoError, etc). And also consider not embedding types like Feature (which is 208 bytes) into the enum fields (maybe).

[lnicola]

We could also box the error details (but I assume clippy already suggests that).

[urschrei]

Consider also #176 and #180

[rmanoka]

Error enum and serde_jso are at loggerheads. Iiuc, Serde json will expect all our semantic errors to be String s.