Better type diff highlight for type variables

[chenglou]

This highlight is misleading. Because it's very possible that 'a is indeed category list. We should put it in another color or something, to indicate that the focus should be on the first argument.

We can leverage the extra type info (that's also parsed out below, if any) and completely skip our custom diffing algorithm in this case, and maybe in the general case too, to only color string list (and the corresponding string -> string list).

In general, I want to somehow show and explain type variables better.

(The actual issue here is that here I've mis-curried by forgetting to provide an extra parameter. Probably needs better message too)

@bsansouci @jordwalke