config.json: Add difficulties

[petertseng]

As noted in #195, difficulty rating is hard.

This commit proposes a difficulty rating scale that should be completely
unambiguous, requiring no judgment:

In the last version of problems.md before it was rewritten in #242:

• Everything in "Introduction" stays at 1.
• Everything in "Getting Rusty" gets a 4.
• Everything in "Rust Gets Strange" gets a 7.
• Everything in "Putting it all Together" gets a 10.

It is acknowledged that this is not a perfect difficulty scheme. See,
for example, how many exercises are at 4.
However, it is more accurate than leaving them all at 1 because it is
generally true that those exercises in "Getting Rusty" are more
difficult than those in "Introduction".
It is a starting point from which further adjustments can be made.
I posit that there is little motivation to adjust further until the
difficulty ratings actually show up on the website, since until then
they are just arbitrary numbers with no meaning.

Closes #179.

[IanWhitney]

it is better than leaving them all at 1

Why?

[petertseng]

Commit message updated.

However, it is more accurate than leaving them all at 1 because it is
generally true that those exercises in "Getting Rusty" are more
difficult than those in "Introduction".

This of course only answers the "why is it better?" question if it is true that we care about the accuracy of the difficulty reported in config.json. This argument can fall flat for at least two reasons (and more that I have not thought of):

1. If we don't care about accuracy. One could rightly argue that we have no reason to care since it doesn't show up anywhere.
2. If we care about accuracy to the extent that an inaccurate measure is worse than making no attempt.

Full disclosure: I think I've made clear that I actually do not care. By extension, this makes me absolutely indifferent to this PR.

[IanWhitney]

A guess is a fine place to start. Maybe later we'll learn something new about difficulty that will improve our guesses. LGTM.