BF F#: use mutable types for Tape and Printer: (#305)
BF F#: use mutable types for Tape and Printer: (#305)

- mutability gives ~ 20s boost for mandel.b test
kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks,kostya/benchmarks