Add changelog entry

Author: kinke

changelog/UniqueTypeInfoNames.md

@@ -0,0 +1,14 @@
+TypeInfo names for aggregates are fully qualified and hence unique now
+
+Previously, template arguments weren't fully qualified; they now are,
+implying longer names in that case.
+
+`TypeInfo_Struct` instances now store the (potentially significantly shorter)
+mangled name only and demangle it lazily on the first `name` or `toString()`
+call (with a per-thread cache). So if you only need a unique string per
+struct TypeInfo, prefer `mangledName` over computed `name` (non-`@nogc` and
+non-`pure`).
+
+**Related breaking change**: `TypeInfo.toString()` isn't `pure` anymore to
+account for the `TypeInfo_Struct` demangled name cache.
+`TypeInfo_Class.toString()` and others are still `pure`.