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/gc.hh

@@ -20,6 +20,7 @@ enum Tag {
     tEnv,
     tWithExprEnv,
     tWithAttrsEnv,
+    tContext,
 
     // Value tags
     tInt,