Inline the common case of link for Set and Map

[meooow25]

I'm seeing some decent improvements in union, difference, etc. on inlining the no-rebalance Bin-Bin case of link

containers/containers/src/Data/Map/Internal.hs

Lines 4019 to 4025 in 82c21f0

	link :: k -> a -> Map k a -> Map k a -> Map k a
	link kx x Tip r = insertMin kx x r
	link kx x l Tip = insertMax kx x l
	link kx x l@(Bin sizeL ky y ly ry) r@(Bin sizeR kz z lz rz)
	| delta*sizeL < sizeR = balanceL kz z (link kx x l lz) rz
	| delta*sizeR < sizeL = balanceR ky y ly (link kx x ry r)
	| otherwise = bin kx x l r

This is just like #1053, that the common case is one where no balancing is required, and making sure that's fast pays off.

I'll prepare a PR for it.

While I'm here, I'm going to change link to delegate to left-only and right-only variants of link. This better represents what's going on, i.e. link never switches directions. I've thought about this but didn't have a good reason to touch the code before.

[meooow25]

This isn't a big win as I first thought.

There are significant improvements (20-30%) but mostly in benchmarks where the two trees don't have a large intersection (and thus the operation is pretty fast anyway), and less (3-5%) otherwise. I'll try to understand this better before making a PR.