Use less c_str() in the evaluator, and other cleanups

It is better to avoid null termination for performance and memory
safety, wherever possible.

These are good cleanups extracted from the Pascal String work that we
can land by themselves first, shrinking the diff in that PR.

Co-Authored-By: Aspen Smith <root@gws.fyi>
Co-Authored-By: Sergei Zimmerman <sergei@zimmerman.foo>

src/libexpr-tests/value/value.cc

@@ -1,6 +1,7 @@
 #include "nix/expr/value.hh"
 
 #include "nix/store/tests/libstore.hh"
+#include <gtest/gtest.h>
 
 namespace nix {
 
@@ -22,4 +23,21 @@ TEST_F(ValueTest, vInt)
     ASSERT_EQ(true, vInt.isValid());
 }
 
+TEST_F(ValueTest, staticString)
+{
+    Value vStr1;
+    Value vStr2;
+    vStr1.mkStringNoCopy("foo");
+    vStr2.mkStringNoCopy("foo");
+
+    auto sd1 = vStr1.string_view();
+    auto sd2 = vStr2.string_view();
+
+    // The strings should be the same
+    ASSERT_EQ(sd1, sd2);
+
+    // The strings should also be backed by the same (static) allocation
+    ASSERT_EQ(sd1.data(), sd2.data());
+}
+
 } // namespace nix