feature: added exercise to "traits" chapter

[abhi3700]

This is an attempt to add 1 exercise in "traits" chapter.

The objective is to let the developer learn these:

• "How to implement at function that can return incompatible types?"
• "How to make the same function printable on console?"

Looking forward to getting feedback on this.

[poneciak57]

Exercise traits6 is unnecessarily large, tests seems redundant here.

[abhi3700]

Exercise traits6 is unnecessarily large, tests seems redundant here.

I can add ?? in tests.

Or I just have only ‘traits7.rs’

[poneciak57]

I think u can just delete tests and add main function with simple showcase.

[abhi3700]

I think u can just delete tests and add main function with simple showcase.

ok followed

[abhi3700]

Can this be merged?

[shadows-withal]

This seems to me like too much of a jump between complexity in exercises (e.g. I don't think we should be touching dyn at this point, or Box before the exercise in which it's introduced).