I think this is okay, the main thing to protect against is new versions
of the format that we don't know about yet.
This happens because an .S file compiled into libc.so has version 2
instead of version 4 like everything else.
Fixes#4491.
I originally defined the bytes() method for the String class, because it
made it obvious that it's a span of bytes instead of span of characters.
This commit makes this more consistent by defining a bytes() method when
the type of the span is known to be u8.
Additionaly, the cast operator to Bytes is overloaded for ByteBuffer and
such.
