Change on-click behavior of detail buttons in the eval timeline about primer-app HOT 1 OPEN

On a related note, I think I have an idea about how to resolve the tension between hackworthltd/vonnegut#622 and hackworthltd/vonnegut#623. I want to write this down before I forget it, but I'm tired and not thinking straight, so I reserve the right to ask you all to ignore this later :)

hackworthltd/vonnegut#622 proposed to draw a line or arrow between eval detail buttons in the eval timeline, in order to indicate to the student where they are in the eval timeline. I objected to this and so instead we went with hackworthltd/vonnegut#623.

After trying out eval mode for some pretty long sequences, I think that we will definitely need the behavior described in this PR, where students can click around in the eval timeline and jump between steps. One problem with that is that it would mean that you'd need to double-click the same button in order to see the details. So we're aliasing multiple views onto the same button, as evidenced by the "one click to jump, two to see the transition" behavior described in this the original post of this PR.

I propose the following, instead: we should render each evaluation step as a roundrect button, and each reduction step as an arrow between the evaluation steps. Above each arrow, show the rule that was applied. (I will draw this up in Figma when I have time.)

Then, clicking on a roundrect will jump to that eval step, and clicking on any arrow will show its reduction details.

We could then do fun things with the step forward/backward buttons, like have 2 backwards buttons and 2 forwards buttons (e.g., ">" and ">>"). The "big step" button would just jump to the next/previous eval step. The "small step" button could possibly render the details right there in the main view: e.g., for λ reduction, first identify/outline the lambda, then the body, then the argument; and finally, shuffle the boxes around to show how the argument becomes the value of the let. (The backwards small-step button could "un-reduce" and put things back where they started... I think?)

from primer-app.