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".
The expression `sometimes & true` now evaluates to `true` 50% of the
time and false 50% of the time, rather than throwing a type error
because `sometimes` is not a bool.