Change unlocking order step 1

[emcoding]

This:

• makes Hello World unlock a few basic exercises, (by changing 'unlocked_by: null to hello-world)
• make all other exercises currently unlocked_by: null move up one or two levels ( unlocked either by TwoFer or by Hamming).

In #518 the request was to restrict the number of unlocked_by: null to 4 or 5 exercises.
Am I right that unlocked_by: hello-world has the same effect in the track? I used that, to make it more explicit.

This PR is an intermediate step in the Ruby Overhaul™.
As a temporary step, all the other exercises that used_to be unlocked_by: null are now moved up 1 or 2 levels, so either by TwoFer for the lowest difficulties, or Hamming, for most others, and some with the highest difficulties quite arbitrary by later core exercises.

I also lowered the difficulty for a few exercises, but they should still be in the same 'easy' category.

[Insti]

Cool thanks. I'm going to have to leave it to someone else to be responsible for merging this as I don't think I should be involved in V2 track organisation.

@hilary seems to have been following V2 closely and I think has the right permissions here.

Or you could try and track down one of the other maintainers

[kotp]

This looks good to me, @hilary how about you?

[hilary]

Looks good to me.