Add EncodeConvert to implementation

[hypirion]

We have edn.AddTagFn and edn.AddTagStruct which makes it possible to convert any tagged EDN value into any untagged value.

edn.EncodeConvert would complement edn.AddTagFn, taking in a function of one argument that returns two arguments: The value to actually encode, and an error if something went wrong.

edn.EncodeAsTag would complement edn.AddTagStruct in the same fashion.

In this way, people can use other people's libraries without having to make wrapper types for every type they export. For example, attaching a definition for time.Time would look like this:

edn.EncodeConvert(func(t time.Time) (edn.Tag, error) {
    return edn.Tag{"inst", t.Format(time.RFC3339Nano)}, nil
})