Add "nix eval" command

This replaces "nix-instantiate --eval". The result is evaluated
strictly since this seems more useful.

src/nix/eval.cc

@@ -0,0 +1,44 @@
+#include "command.hh"
+#include "common-args.hh"
+#include "installables.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "eval.hh"
+#include "json.hh"
+#include "value-to-json.hh"
+
+using namespace nix;
+
+struct CmdEval : MixJSON, MixInstallables
+{
+    std::string name() override
+    {
+        return "eval";
+    }
+
+    std::string description() override
+    {
+        return "evaluate a Nix expression";
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto state = getEvalState();
+
+        auto jsonOut = json ? std::make_unique<JSONList>(std::cout) : nullptr;
+
+        for (auto & i : installables) {
+            auto v = i->toValue(*state);
+            if (json) {
+                PathSet context;
+                auto jsonElem = jsonOut->placeholder();
+                printValueAsJSON(*state, true, *v, jsonElem, context);
+            } else {
+                state->forceValueDeep(*v);
+                std::cout << *v << "\n";
+            }
+        }
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdEval>());