Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/os-string.hh

@@ -0,0 +1,52 @@
+#pragma once
+///@file
+
+#include <optional>
+#include <string>
+#include <string_view>
+
+namespace nix {
+
+/**
+ * Named because it is similar to the Rust type, except it is in the
+ * native encoding not WTF-8.
+ *
+ * Same as `std::filesystem::path::value_type`, but manually defined to
+ * avoid including a much more complex header.
+ */
+using OsChar =
+#if defined(_WIN32) && !defined(__CYGWIN__)
+    wchar_t
+#else
+    char
+#endif
+    ;
+
+/**
+ * Named because it is similar to the Rust type, except it is in the
+ * native encoding not WTF-8.
+ *
+ * Same as `std::filesystem::path::string_type`, but manually defined
+ * for the same reason as `OsChar`.
+ */
+using OsString = std::basic_string<OsChar>;
+
+/**
+ * `std::string_view` counterpart for `OsString`.
+ */
+using OsStringView = std::basic_string_view<OsChar>;
+
+std::string os_string_to_string(OsStringView path);
+
+OsString string_to_os_string(std::string_view s);
+
+/**
+ * Create string literals with the native character width of paths
+ */
+#ifndef _WIN32
+#  define OS_STR(s) s
+#else
+#  define OS_STR(s) L##s
+#endif
+
+}