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.cc

@@ -244,7 +244,7 @@ static Strings parseNixPath(const string & in)
 }
 
 
-EvalState::EvalState(const Strings & _searchPath)
+EvalState::EvalState(const Strings & _searchPath, ref<StoreAPI> store)
     : sWith(symbols.create("<with>"))
     , sOutPath(symbols.create("outPath"))
     , sDrvPath(symbols.create("drvPath"))
@@ -262,6 +262,7 @@ EvalState::EvalState(const Strings & _searchPath)
     , sColumn(symbols.create("column"))
     , sFunctor(symbols.create("__functor"))
     , sToString(symbols.create("__toString"))
+    , store(store)
     , baseEnv(allocEnv(128))
     , staticBaseEnv(false, 0)
 {