Re-rank and re-order exercises

[ryanplusplus]

This definitely isn't perfect, but I think it's a pretty significant improvement over our current state. What I did was:

• Re-assigned difficulties based on gut feel (from my own experience and by looking at our solutions), completion percentages (see https://exercism.org/tracks/c/build), and topics covered (for example, anything with memory management was bumped up to at least a 4 or 5... we had several at 1 and 2 😐)
• Ordered all of the exercises by difficulty
• Within the lower difficulty levels, I ordered exercises such that not too much was introduced at a time.

As a next step, I think we can pick a difficulty cut-off point and start adding more complete skeletons to earlier exercises. This would address:

• More documentation to Grade School #911
• Inconsistency in headers provided #740

Fixes #971

[wolf99]

Looks good!
I guess we try it and see if the stats improve for students' linear progression in the track
Some small question-ey comments only 🙂