Register all PrimOps via the Info structure

This will allow documenting them (in later commits).

Note that we keep the old constructor even if it is no longer used by
Nix code, because it is used in tests/plugins/plugintest.cc, which
suggests that it might be used by some external plugin.

src/libexpr/primops/fetchTree.cc

@@ -194,7 +194,11 @@ static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args,
 }
 
 // FIXME: document
-static RegisterPrimOp primop_fetchTree("fetchTree", 1, prim_fetchTree);
+static RegisterPrimOp primop_fetchTree({
+    .name = "fetchTree",
+    .arity = 1,
+    .fun = prim_fetchTree
+});
 
 static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v,
     const std::string & who, bool unpack, std::string name)