Add 'format' concept & exercise

[verdammelt]

The text-formatting stub concept was removed before launch in #443.

This concept should teach the basics of how to use 'format'. Would be good to allow it to be a relatively early concept so it should not delve into streams as those are likely to be a much later concept.

Perhaps the exercise would just use nil stream for creating strings.

There will likely be a whole tree of 'format' concepts as it is a complex topic.

NOTE: the roman-numeral exercise will both rely upon format and practice it. Perhaps roman numeral effectively becomes part of the format concept exercise?

[verdammelt]

"Basics of format" probably should include ~A/~S/~W, ~D, &/%. Could include some more - but should be limited so it is 1) not too big 2) can be kept as an early concept.

[verdammelt]

Reviewing ~A vs. ~S vs. ~W doesn't show an easy way of including both ~S and ~W as normally they are going to have pretty much the same output. (I'm making the assumption that this basic exercise will not be going into @ or : or the min/max col type arguments to the directives.

However ~A vs. ~S/~W is something that can be taught and tested for. ~A is for human consumption while the others are for (generally) machine consumption.

Might be reasonable to leave ~D out of this as ~A/~S can handle numbers just fine.

That seems to leave this exercise being about ~A/~S and &/% which will make for a small but useful concept/exercise.

I think the concept should be called something like formatting-basic.

[verdammelt]

As a contrary viewpoint - perhaps the printer should be introduced rather than formatted output? After all formatted output is based upon the printer.

That being said I think most students will have some familiarity with languages that have some function that is in some way derived from printf, that is, a function that does both string concatenation and output to a stream.

So I still think teaching the student about format first vs write or one of the print functions makes sense.