Style fixes

In particular, use std::make_shared and enumerate(). Also renamed some
fields to fit naming conventions.

src/libcmd/command.hh

@@ -275,6 +275,7 @@ void printClosureDiff(
 
 void runRepl(
     ref<EvalState> evalState,
-    const Expr &expr,
+    const Expr & expr,
     const std::map<std::string, Value *> & extraEnv);
+
 }