provide our own hashset implementation

[cparser] / libfirm

diff --git a/libfirm b/libfirm
index 78b94dd..f2ad275 160000 (submodule)
--- a/libfirm
+++ b/libfirm
@@ -1 +1 @@
-Subproject commit 78b94dd9a1d6aee17e0dfb4218a49ef17531d680
+Subproject commit f2ad275bb3619165357b369ac083ae2f73d5a543