Extracted the preupdate_hook to a separate cargo feature.

Moved hooks and preupdate_hook into their own modules inside hooks.rs
Also created an initial way to access the functions that are available
during the callback.

Cargo.toml

@@ -37,7 +37,8 @@ trace = ["libsqlite3-sys/min_sqlite_version_3_6_23"]
 bundled = ["libsqlite3-sys/bundled", "modern_sqlite"]
 buildtime_bindgen = ["libsqlite3-sys/buildtime_bindgen"]
 limits = []
-hooks = ["libsqlite3-sys/preupdate_hook"]
+hooks = []
+preupdate_hook = ["libsqlite3-sys/preupdate_hook"]
 i128_blob = ["byteorder"]
 sqlcipher = ["libsqlite3-sys/sqlcipher"]
 unlock_notify = ["libsqlite3-sys/unlock_notify"]