Divide by zero and add, subtract, or multiply with overflow are all
caught now and reported as an ArithmeticError, rather than causing the
interpreter to panic.
Renamed ControlFlow -> HaltStatus because that's what the enum really
is -- a status on why something halted. Also reviewed `interpret.rs`'s
documentation and fixed a few things that were out of date.
Added variable declaration, `if` statements, `loop` statements,
variable assignment, and variable banning to go along with
printing (which was already implemented). We still need function
declarations, brainfuck declarations, function calls, and the control
flow operators "break" and "hopback".
Alex Bethel
