InteractiveInterpreter, which handles parsing and interpreter state
but doesn't know deal with buffering or prompting or input file
naming.  And a derived class, InteractiveConsole, which adds buffering
and prompting and supports setting the filename once.  Also tweak the
algorithm in compile_command() a bit so that input consisting of all
blank lines or comments always succeeds immediately, and note the fact
that apart from SyntaxError it can also raise OverflowError.