Store contexts as symbols

This provides some deduplication since most contexts are used multiple
times.

Also, store singleton contexts directly in the Value object. This
saves a 16-byte Context object. This is useful because the vast
majority of contexts are singletons, e.g. 23723 out of 26138 in a
NixOS 19.03 system configuration.

src/libexpr/value-to-xml.cc

@@ -70,7 +70,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
         case tString:
             /* !!! show the context? */
-            copyContext(v, context);
+            v.getContext(context);
             doc.writeEmptyElement("string", singletonAttrs("value", v.string.s));
             break;