Split the server into three crates (#56)

This will make it easier to build variations on the server, or embed it
into larger projects.

core/src/lib.rs

@@ -0,0 +1,32 @@
+//! This crate implements the core logic of the taskchampion sync protocol.
+//!
+//! This should be considered a reference implementation, with [the protocol
+//! documentation](https://gothenburgbitfactory.org/taskchampion/sync-protocol.html). representing
+//! the authoritative definition of the protocol. Other implementations are encouraged.
+//!
+//! This crate uses an abstract storage backend. Note that this does not implement the
+//! HTTP-specific portions of the protocol, nor provide any storage implementations.
+//!
+//! ## API Methods
+//!
+//! The following API methods are implemented. These methods are documented in more detail in
+//! the protocol documentation.
+//!
+//! * [`add_version`]
+//! * [`get_child_version`]
+//! * [`add_snapshot`]
+//! * [`get_snapshot`]
+//!
+//! Each API method takes:
+//!
+//! * [`StorageTxn`] to access storage. Methods which modify storage will commit the transaction before returning.
+//! * [`ServerConfig`] providing basic configuration for the server's behavior.
+//! * `client_id` and a [`Client`] providing the client metadata.
+
+mod inmemory;
+mod server;
+mod storage;
+
+pub use inmemory::*;
+pub use server::*;
+pub use storage::*;