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-json.cc

@@ -27,7 +27,7 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
 
         case tString:
-            copyContext(v, context);
+            v.getContext(context);
             out.write(v.string.s);
             break;