Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Applications/Spreadsheet/Cell.cpp

@@ -162,7 +162,7 @@ DeprecatedString Cell::source() const
     if (m_kind == Formula)
         builder.append('=');
     builder.append(m_data);
-    return builder.to_string();
+    return builder.to_deprecated_string();
 }
 
 // FIXME: Find a better way to figure out dependencies