libexpr: fully separate evaluator and running evaluation

this finally gives us a witness type we can use to prove that a certain
call graph subtree can't be used in kj promises using only a single new
assumption: if EvalState& is never held as a reference member of a type
and instead only ever passes as an argument or held on the stack we can
be certain that anything that has access to en EvalState ref must never
be run inside a promise and, crucially, that anything that doesn't have
access to an EvalState& *can* be run inside a promise without problems.

Change-Id: I6c15ada479175ad7e6cd3e4a729a5586b3ba30d6

lix/libexpr/primops/fetchTree.cc

@@ -15,7 +15,7 @@
 namespace nix {
 
 void emitTreeAttrs(
-    EvalState & state,
+    Evaluator & state,
     const fetchers::Tree & tree,
     const fetchers::Input & input,
     Value & v,
@@ -196,7 +196,7 @@ static void fetchTree(
 
     state.ctx.paths.allowPath(tree.storePath);
 
-    emitTreeAttrs(state, tree, input2, v, params.emptyRevFallback, false);
+    emitTreeAttrs(state.ctx, tree, input2, v, params.emptyRevFallback, false);
 }
 
 static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)