Remove std::string alias (for real this time)

Also use std::string_view in a few more places.

src/libexpr/value-to-xml.cc

@@ -9,7 +9,7 @@
 namespace nix {
 
 
-static XMLAttrs singletonAttrs(const string & name, const string & value)
+static XMLAttrs singletonAttrs(const std::string & name, const std::string & value)
 {
     XMLAttrs attrs;
     attrs[name] = value;