Change debug::puts to use raw char* and copy

In order to handle the self-assign case, string::operator= moves the
string, which may copy the string in reverse. The MGBA string register
does not allow this I believe. In order to avoid this, it now uses a raw
character buffer and is copied using string_view::copy.
string_view::copy will always copy the string forward.

src/debug.cpp

@@ -31,11 +31,17 @@ static volatile uint16_t& flags = *reinterpret_cast<uint16_t*>(0x4FFF700);
  *
  * Max length of 0x100 (256) including null terminator.
  */
-static mtl::string_ext string(reinterpret_cast<char*>(0x4FFF600), 256);
+static char* string = reinterpret_cast<char*>(0x4FFF600);
+constexpr static size_t string_len = 255;
 }; // namespace reg
 
 void puts(const mtl::string_view& msg, level lvl) {
-	reg::string = msg;
+	size_t count = msg.length();
+	if (count > reg::string_len) { count = reg::string_len; }
+
+	// Must be copied manually because the MGBA string register does
+	// not allow copying in reverse (I think)
+	msg.copy(reg::string, count);
 	reg::flags = lvl | 0x100;
 }