Add changelog entries

Author: arcz

CHANGELOG.md

@@ -5,9 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Fixed
+
+- Arithmetic overflow in concrete `SAR` edge case
+- Unnecessary application of `CopySlice` in concrete edge case
+
 ## [0.50.1] - 2022-12-29
 
-## Fixed
+### Fixed
 
 - `hevm exec` no longer fails with `hevm: No match in record selector smttimeout`
 - the `gas`, `gaslimit`, `priorityfee`, and `gasprice` cli options are now respected