[rt/aaA] Add _aaNew function to initialize AA

Part of https://issues.dlang.org/show_bug.cgi?id=10535.

Author: ntrel

src/rt/aaA.d

@@ -485,6 +485,14 @@ pure nothrow @nogc unittest
 // API Implementation
 //------------------------------------------------------------------------------
 
+/// Allocate new AA implementation.
+extern (C) AA _aaNew(const TypeInfo_AssociativeArray ti)
+{
+    AA aa;
+    aa.impl = new Impl(ti);
+    return aa;
+}
+
 /// Determine number of entries in associative array.
 extern (C) size_t _aaLen(scope const AA aa) pure nothrow @nogc
 {