deprecate / replace `sml_transport` API

[r00t-]

the sml_transport API is far too primitive to be useful,
sml_transport_read() does not have any provision to keep state between invocations,
and no way to return more detailed information. (it can only return an int, and only write to stderr (see #17 )

• we already re-defined the API so that a 0 return means "error": handle read errors and eof #51
• we can not generate a warning (neither by returning to the caller, nor via stderr - because we can't even keep state) if the data read is being ignored when no start sequence is found (see e.g. no logging vzlogger#633 (comment) )