AK: Rename span() to bytes() when appropriate.

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.

Userland/base64.cpp

@@ -75,6 +75,6 @@ int main(int argc, char** argv)
         return 0;
     }
 
-    auto encoded = encode_base64(buffer.span());
+    auto encoded = encode_base64(buffer);
     printf("%s\n", encoded.characters());
 }