The new Attached_dataspace complements the existing Attached_*
utilities with a simple version that can be used with any kind of
dataspaces. It may be even useful as a common base type for the other
variants. For example, this patch simplifies Attached_rom_dataspace
and removes the Terminal::Client::Io_buffer.
The new 'Uart::Session' interface is an extension of the
'Terminal::Session' interface that allows for configuring UART-specific
parameters, i.e., the baud rate.
