added Isabelle_Process.input_bytes, which avoids the somewhat slow Standard_System.string_bytes (just in case someone wants to stream raw data at 250MB/s);

do not buffer fifo streams here -- done in Isabelle_Process; removed misleading comments -- blocking on open is not reliable on non-standard systems (Cygwin);

explicit Document.State value, instead of individual state variables in Session, Command, Document; Session.snapshot: pure value based on Document.State; Document.edit_texts: no treatment of state assignment here;