[Mammoth/Teton] Add an OpenFile interface and use it to load a font.

lib/mammoth/file/file.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <glacier/memory/move.h>
+#include <glacier/string/string_view.h>
+
+#include "mammoth/util/memory_region.h"
+
+namespace mmth {
+
+class File {
+ public:
+  static File Open(glcr::StringView path);
+
+  glcr::StringView as_str();
+
+  void* raw_ptr();
+  uint8_t* byte_ptr();
+
+ private:
+  OwnedMemoryRegion file_data_;
+
+  File(OwnedMemoryRegion&& file) : file_data_(glcr::Move(file)) {}
+};
+
+}  // namespace mmth