Add Rustlings fast-track

[jpedrick]

I've been learning rust for a few months now, but I'm still benefitting from Rustlings as there are details covered that I haven't learned yet. I'm probably not the only one who found the cost/benefit (time/(learning/fun)) ratio a bit high(admittedly, I should have walked through rustlings a couple months ago.) I'm proposing an option to solve a single fast-track exercise that contains all the concepts for each chapter, potentially allowing users to switch back and forth within rustings watch. If a user finds the fast-track too daunting, they can switch back to the standard track and vice versa. When testing if the chapter is completed, first check the fast-track exercise and if it runs correctly, skip the rest of the chapter.

Welcome to watch mode! You can type 'help' to get an overview of the commands you can use here.
help
Commands available to you in watch mode:
  hint  - prints the current exercise's hint
  [fast-track/standard-track] - switch to fast-track exercises / switch to standard-track exercises
  clear - clears the screen
  quit  - quits watch mode
  help  - displays this help message

Watch mode automatically re-evaluates the current exercise
when you edit a file's contents.

[shadows-withal]

Interesting idea! I'll keep it in mind, I've been meaning to take another look at the progression some time.