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.
Currently, contains one function: compile_command(), which helps
determining whether a source string is complete, incomplete or in
error. This is useful when writing your own version of the Python
read-eval-print loop.
