Cache path info lookups in SQLite

This re-implements the binary cache database in C++, allowing it to be
used by other Store backends, in particular the S3 backend.

src/libutil/hash.hh

@@ -7,6 +7,9 @@
 namespace nix {
 
 
+MakeError(BadHash, Error);
+
+
 enum HashType : char { htUnknown, htMD5, htSHA1, htSHA256, htSHA512 };
 
 
@@ -26,12 +29,15 @@ struct Hash
 
     HashType type;
 
-    /* Create an unusable hash object. */
+    /* Create an unset hash object. */
     Hash();
 
     /* Create a zero-filled hash object. */
     Hash(HashType type);
 
+    /* Check whether a hash is set. */
+    operator bool () const { return type != htUnknown; }
+
     /* Check whether two hash are equal. */
     bool operator == (const Hash & h2) const;
 
@@ -52,12 +58,16 @@ struct Hash
     {
         return (hashSize * 8 - 1) / 5 + 1;
     }
+
+    std::string to_string(bool base32 = true) const;
 };
 
 
 /* Convert a hash to a hexadecimal representation. */
 string printHash(const Hash & hash);
 
+Hash parseHash(const string & s);
+
 /* Parse a hexadecimal representation of a hash code. */
 Hash parseHash(HashType ht, const string & s);