* Renamed normalise.cc' -> build.cc', `storeexprs.cc' ->

`derivations.cc', etc.
* Store the SHA-256 content hash of store paths in the database after
  they have been built/added.  This is so that we can check whether
  the store has been messed with (a la `rpm --verify').
* When registering path validity, verify that the closure property
  holds.

src/libexpr/nixexpr.cc

@@ -1,5 +1,5 @@
 #include "nixexpr.hh"
-#include "storeexpr.hh"
+#include "derivations.hh"
 
 
 #include "nixexpr-ast.hh"