Add a custom `Cursor` type to synom.

This type allows for efficiently traversing the input buffer once it has
been transformed, and allows for None-delimited sequences to be
efficiently implicitly ignored when they are not needed.

This module contains thoroughly commented unsafe code.
diff --git a/synom/src/lib.rs b/synom/src/lib.rs
index 8b57b3d..f656e10 100644
--- a/synom/src/lib.rs
+++ b/synom/src/lib.rs
@@ -43,6 +43,7 @@
 pub mod delimited;
 pub mod tokens;
 pub mod span;
+pub mod cursor;
 
 /// A cursor into a Vec<TokenTree>.
 ///