Generic schemes use foldl

[isovector]

I don't know enough to have a strong opinion here, but the HS community seems to be firmly behind using foldl' over foldl. However, this library uses foldl in a number of places:

syb/src/Data/Generics/Schemes.hs

Line 122 in f741a43

go x = foldl k (f x) (gmapQ go x)

syb/src/Data/Generics/Schemes.hs

Line 132 in f741a43

else foldl k v (gmapQ go x)

syb/src/Data/Generics/Schemes.hs

Line 140 in f741a43

go s x = foldl f r (gmapQ (go s') x)

syb/src/Data/Generics/Schemes.hs

Line 202 in f741a43

. foldl unJust []

This seems likely to be an easy performance win