Better layout algorithm for map children

[mkremins]

At the moment, maps are rendered very naïvely with regards to layout. Instead, an algorithm something like the following should be used:

• If the entire rendered map can fit on a single line, layout children without line breaks.
• If multiple lines are needed to accommodate all of the map's children, attempt to fit each key-value pair on its own line and insert line breaks between key-value pairs.
• If a given key-value pair cannot fit on a single line, insert an additional break between its key and its value.

Ideally, the layout algorithm should also attempt to vertically align multiline maps' keys and values into columns for clarity.

[lypanov]

It'd be great if forms taking pairwise parameters (eg cond / condp) could also get the same layout.

[mkremins]

This is definitely also something I'd like to have. Maybe it'll be possible to extend the mini-DSL I'm using for sequence layout into something that's smart enough to implement this sort of behavior. Properly implemented, that'd allow users to provide specialized layout options for other macros as well.