Let user reattempt solving an arbitrary step in the tutorial

[anton-trunov]

I cannot figure out a way of redefining a solution to a chosen step of the tutorial if I have already found a solution and the step is marked [solved]. E.g. I find a solution from the first principles and and then I want to check another more elegant one.

[evinism]

Hmm, would just enabling the win condition for the problem you're currently looking at be a sufficient enough user interface? Or do you want a more explicit "retry" option?

[anton-trunov]

"Retry" sounds good! Or if I redefine the variable (like the variable E of problem 28) I'd like to have my solution re-checked. E.g. right now it seems that redefining E to be something deliberately wrong (like E := λx.x) does nothing: the problem is still [solved].

[evinism]

interim version implemented here, this just checks win condition for your current problem https://github.com/evinism/lambda-explorer/pull/57