optionally return string_view from coerceToString

we'll retain the old coerceToString interface that returns a string, but callers
that don't need the returned value to outlive the Value it came from can save
copies by using the new interface instead. for values that weren't stringy we'll
pass a new buffer argument that'll be used for storage and shouldn't be
inspected.

src/libexpr/eval.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "attr-set.hh"
+#include "types.hh"
 #include "value.hh"
 #include "nixexpr.hh"
 #include "symbol-table.hh"
@@ -251,7 +252,7 @@ public:
        string.  If `coerceMore' is set, also converts nulls, integers,
        booleans and lists to a string.  If `copyToStore' is set,
        referenced paths are copied to the Nix store as a side effect. */
-    string coerceToString(const Pos & pos, Value & v, PathSet & context,
+    BackedStringView coerceToString(const Pos & pos, Value & v, PathSet & context,
         bool coerceMore = false, bool copyToStore = true,
         bool canonicalizePath = true);