Added conclude by contradiction construction

[pimotte]

Fixes #31

We are a little bit further in formalizing some exercises for our introduction to proofs course, and we really do want to have this. If you prefer to have it scoped, I'm happy to set that up too.

I'm also not entirely sure about what the deal is with whitespace/division into strings for elab rules, so I might have done something silly there.

[PatrickMassot]

I think the string division stuff in this file is a historical accident which is no longer necessary. But you don’t have to fix it in this PR, being consistent with the surrounding syntax declarations is good.

Please scope those new syntax if this is isn’t causing too much trouble for you because I’m really not convinced this is generally useful (and, as every new syntax, it introduces new parser confusion potential).