Make HashTable work complex types as key

- Add toHash() function for String
- The key type shouldn't match exact for a lookup.
The key type and lookup key type should be comparable.
- Move elements when inserting if passed by value.

source/tanya/container/string.d

@@ -31,6 +31,7 @@ import std.algorithm.mutation : bringToFront, copy;
 import std.algorithm.searching;
 import tanya.algorithm.comparison;
 import tanya.algorithm.mutation;
+import tanya.hash.lookup;
 import tanya.memory;
 import tanya.meta.trait;
 import tanya.meta.transform;
@@ -1614,6 +1615,16 @@ struct String
         assert(s == "Казнить, нельзя помиловать.");
     }
 
+    /**
+     * Calculates the hash value for the string.
+     *
+     * Returns: Hash value for the string.
+     */
+    size_t toHash() const @nogc nothrow pure @safe
+    {
+        return hash(get);
+    }
+
     mixin DefaultAllocator;
 }