Speed up cartesian axes supplyDefaults

From https://github.com/plotly/plotly.js/pull/2527, Plots.supplyDefaults takes > 100ms at 50x50 subplots which feels like an eternity when zooming.

• Getting rid of indexOf into axis lists would be a good start (perhaps experimenting with ES6 Map and Set could be worthwhile)
• Perhaps we could try opening a partial Plots.supplyDefaults pathway, that would only update a subset of the graph’s cartesian axes and their dependencies.

[alexcjohnson]

So I’m thinking of moving this block out of Plots.linkSubplots and into the lsInner subroutine

Sounds reasonable. Another option - not necessarily mutually exclusive, we could do both in fact - would be to make another collation of subplots that maps each (x|y) axis onto the (y|x) axes it makes subplots with. That should substantially reduce the size and complexity of loops like this.

[etpinard]

Referencing https://github.com/plotly/plotly.js/issues/749 - moving away from the “relink everything approach” should speed up supplyDefaults in some scenarios.