nix search: Add a cache

The package list is now cached in
~/.cache/nix/package-search.json. This gives a substantial speedup to
"nix search" queries. For example (on an SSD):

First run: (no package search cache, cold page cache)

  $ time nix search blender
  Attribute name: nixpkgs.blender
  Package name: blender
  Version: 2.78c
  Description: 3D Creation/Animation/Publishing System

  real    0m6.516s

Second run: (package search cache populated)

  $ time nix search blender
  Attribute name: nixpkgs.blender
  Package name: blender
  Version: 2.78c
  Description: 3D Creation/Animation/Publishing System

  real    0m0.143s

2 files changed, 23 insertions, 11 deletions

diff --git a/src/libutil/json.cc b/src/libutil/json.cc
index b8b8ef9c8cca..813b257016e4 100644
--- a/src/libutil/json.cc
+++ b/src/libutil/json.cc
@@ -50,20 +50,22 @@ template<> void toJSON<std::nullptr_t>(std::ostream & str, const std::nullptr_t
 JSONWriter::JSONWriter(std::ostream & str, bool indent)
     : state(new JSONState(str, indent))
 {
-    state->stack.push_back(this);
+    state->stack++;
 }
 
 JSONWriter::JSONWriter(JSONState * state)
     : state(state)
 {
-    state->stack.push_back(this);
+    state->stack++;
 }
 
 JSONWriter::~JSONWriter()
 {
-    assertActive();
-    state->stack.pop_back();
-    if (state->stack.empty()) delete state;
+    if (state) {
+        assertActive();
+        state->stack--;
+        if (state->stack == 0) delete state;
+    }
 }
 
 void JSONWriter::comma()
@@ -121,9 +123,11 @@ void JSONObject::open()
 
 JSONObject::~JSONObject()
 {
-    state->depth--;
-    if (state->indent && !first) indent();
-    state->str << "}";
+    if (state) {
+        state->depth--;
+        if (state->indent && !first) indent();
+        state->str << "}";
+    }
 }
 
 void JSONObject::attr(const std::string & s)
diff --git a/src/libutil/json.hh b/src/libutil/json.hh
index 595e9bbe3491..02a39917fb5c 100644
--- a/src/libutil/json.hh
+++ b/src/libutil/json.hh
@@ -21,11 +21,11 @@ protected:
         std::ostream & str;
         bool indent;
         size_t depth = 0;
-        std::vector<JSONWriter *> stack;
+        size_t stack = 0;
         JSONState(std::ostream & str, bool indent) : str(str), indent(indent) { }
         ~JSONState()
         {
-            assert(stack.empty());
+            assert(stack == 0);
         }
     };
 
@@ -41,7 +41,7 @@ protected:
 
     void assertActive()
     {
-        assert(!state->stack.empty() && state->stack.back() == this);
+        assert(state->stack != 0);
     }
 
     void comma();
@@ -117,6 +117,14 @@ public:
         open();
     }
 
+    JSONObject(const JSONObject & obj) = delete;
+
+    JSONObject(JSONObject && obj)
+        : JSONWriter(obj.state)
+    {
+        obj.state = 0;
+    }
+
     ~JSONObject();
 
     template<typename T>