Add an as_of_root argument to Tree::authentication_path

This requires a user of this API to specify a root of the tree
that identifies the state of the tree at which the user wishes
to construct an authentication path. This must be equal to either
the current root of the tree, or to the root of the tree at a
previous checkpoint.

CHANGELOG.md

@@ -7,6 +7,29 @@ and this project adheres to Rust's notion of
 
 ## [Unreleased]
 
+### Added
+
+- `incrementalmerkletree::bridgetree`:
+  - `Checkpoint::witnessed` returns the set of positions that have been marked with
+    `BridgeTree::witness` while this checkpoint was the current checkpoint.
+
+### Changed
+
+- `incrementalmerkletree`:
+  - `Tree::authentication_path` has been changed to take an `as_of_root` parameter.
+    This allows computation of the authentication path as of previous tree states, in
+    addition to the previous behavior which only allowed computation of the path as of the
+    most recent tree state. The provided `as_of_root` value must be equal to either the
+    current root of the tree, or to the root of the tree at a previous checkpoint. 
+
+### Removed
+
+- `incrementalmerkletree::bridgetree`:
+  - `Checkpoint::rewrite_indices` was an internal utility method that had inadvertently
+    been made a part of the public API.
+
+## [0.3.0-beta.2] - 2022-04-06
+
 ### Added
 - `incrementalmerkletree`:
   - `Tree::get_witnessed_leaf`, to allow a user to query for the leaf value of