fix(treewide): remove unnecessary copying in range for loops

This gets rid of unnecessary copies in range-based-for loops and
local variables, when they are used solely as `const &`.

Also added a fixme comment about a suspicious move out of const,
which might not be intended.

src/libexpr/primops.cc

@@ -4383,7 +4383,7 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 
         for (auto i = begin; i != end; ++i) {
             assert(idx <= 2 * len + 1 - 3);
-            auto match = *i;
+            const auto & match = *i;
 
             // Add a string for non-matched characters.
             list[idx++] = mkString(state, match.prefix());