Eliminate the "store" global variable

Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

src/libexpr/eval.hh

@@ -16,6 +16,7 @@
 namespace nix {
 
 
+class StoreAPI;
 class EvalState;
 
 
@@ -82,6 +83,8 @@ public:
 
     Value vEmptySet;
 
+    const ref<StoreAPI> store;
+
 private:
     SrcToStore srcToStore;
 
@@ -97,7 +100,7 @@ private:
 
 public:
 
-    EvalState(const Strings & _searchPath);
+    EvalState(const Strings & _searchPath, ref<StoreAPI> store);
     ~EvalState();
 
     void addToSearchPath(const string & s, bool warn = false);
@@ -240,6 +243,8 @@ public:
     /* Print statistics. */
     void printStats();
 
+    void realiseContext(const PathSet & context);
+
 private:
 
     unsigned long nrEnvs = 0;
@@ -290,7 +295,4 @@ struct InvalidPathError : EvalError
 #endif
 };
 
-/* Realise all paths in `context' */
-void realiseContext(const PathSet & context);
-
 }