feat: Allow the UndoLog to supplied separately

From rust-lang/rust#69218 it was found that
keeping an individual undo log for each type that need snapshotting
incur noticeable overhead given how often snapshots are done and that
they are in many cases empty.

By separating the log like this, it is possible to store the undo log
and storage separately and only put the together when acting the the
underlying `UnificationTable` or `SnapshotVec`

Author: Markus Westerlind

src/lib.rs

@@ -20,4 +20,5 @@ extern crate log;
 extern crate dogged;
 
 pub mod snapshot_vec;
+pub mod undo_log;
 pub mod unify;