BF functios can now be declared and called from AbleScript code.
Function parameters supplied from AbleScript are serialized manually
into byte sequences using the `BfWriter` trait, and are available from
BF code using the input operator, `,`.
At the moment, BF functios simply write output to stdout, and are
therefore incapable of communicating results to the rest of the
program; this might change in the future if we can get something close
to pass-by-reference working.
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documentation and fixed a few things that were out of date.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
