JMAP Core implementation

Reference: https://jmap.io/spec-core.html
List below is nowhere complete.

- [x] Fundamental types
- [x] Int
- [x] UnsignedInt
- [x] Date
- [x] UTCDate
- [x] Id
- [x] Autodiscovery
- [ ] Structures for base JMAP Core objects
- [x] Session
- [x] Account
- [x] Core Capabilities
- [x] Request-level errors
- [x] Invocation
- [x] Method-level errors
- [x] Decode Invocation "subtypes" using type -> decoder mapping.
- [x] Request
- [x] Response
- [ ] Set-level errors
- [ ] Core request objects
- [ ] Object/get
- [ ] Object/changes
- [ ] Object/set
- [ ] Object/copy
- [ ] Object/query
- [ ] Object/queryChanges
- [x] Core/echo
- [ ] Binary data I/O
- [x] Downloading
- [x] Uploading
- [ ] Blob/Copy method
- [ ] Push
- [ ] StateChange structure
- [ ] PushSubscription structure
- [ ] PushSubscription/get
- [ ] PushSubscription/set
- [ ] Event Source
- [x] Client interface
- [x] Get Session object
- [x] Interface to send request objects directly
- [x] "Call batch builder" interface
- [x] Binary I/O interface
- [ ] Server interface
- [ ] Base backend interface
- [ ] Binary I/O backend interface
- [ ] Method call dispatching