sorting-room: Implement exercise to teach about type-conversion and type-assertion

[jmrunkle]

Part of #1645

This is not complete yet since the introduction should be filled out from details in the concepts so leaving it as "wip", but I wanted to start getting feedback.

[junedev]

FYI I merged the concept MRs so their content is now available to be included here.

[bitfield]

I dare say you've already thought about this, but it's worth mentioning that generics will be shipping soon, perhaps as soon as December. We'll need to fit that into our syllabus along with interfaces, won't we? I don't know if you already have an exercise planned for this.

[junedev]

@bitfield I haven't looked at the final proposal that made it through yet. Do you see an issue with just adding a separate exercise on generics somewhat further down the concept tree?

[jmrunkle]

FYI I merged the concept MRs so their content is now available to be included here.

Added material from the concept docs.

[jmrunkle]

Ready for another look whenever you are ready @junedev

[bitfield]

My blog post summarises roughly what students will need to know. Yes, we need to tackle interfaces before generics—you could think of generics as further loosening the constraints on what kind of things a function can accept. Instead of just being constrained by method sets, for example io.Reader, they can now be constrained by type lists (all integer types, for example) or by attributes like comparable or constraints.Ordered. So I'm sure we could come up with a similar exercise along those lines.

[junedev]

Exercise looks good now! Left 2 more tiny comments, feel free to merge whenever you see fit.