libexpr: use pascal strings for eval

this has no performance impact in any benchmarks we've run. nul bytes
are still used as implicit truncation points in many places all over:
rejecting them in all locations that treat them as a string end point
requires large changes such as using a proper path library everywhere

Change-Id: I936158bd435f6abf009a689adfbc24496262c578

tests/unit/libexpr-support/tests/libexpr.hh

@@ -112,7 +112,7 @@ namespace nix {
             if (arg.type() != nPath) {
                 *result_listener << "Expected a path got " << arg.type();
                 return false;
-            } else if (std::string_view(arg.string().content) != p) {
+            } else if (arg.string().content->str() != p) {
                 *result_listener << "Expected a path that equals \"" << p
                                  << "\" but got: " << arg.path();
                 return false;