Added missing v1.7.3 CHANGELOG (#2356)

Author: MatthewDaggitt

CHANGELOG/v1.7.3.md

@@ -0,0 +1,18 @@
+Version 1.7.3
+=============
+
+The library has been tested using Agda 2.6.3 & 2.6.4.
+
+* To avoid _large indices_ that are by default no longer allowed in Agda 2.6.4,
+  universe levels have been increased in the following definitions:
+  - `Data.Star.Decoration.DecoratedWith`
+  - `Data.Star.Pointer.Pointer`
+  - `Reflection.AnnotatedAST.Typeₐ`
+  - `Reflection.AnnotatedAST.AnnotationFun`
+
+* The following aliases have been added:
+  - `IO.Primitive.pure` as alias for `IO.Primitive.return`
+  - modules `Effect.*` as aliases for modules `Category.*`
+
+  These allow to address said objects with the new name they will have in v2.0 of the library,
+  to ease the transition from v1.7.3 to v2.0.