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.
This fixes the issue with the exported data having a leading zero,
causing RSA::encrypt to trim the block down, and ruining the encryption.
Fixes#2691 :^)
