Implemement foldr, foldl', sum and product in Foldable Tree

[phadej]

Addresses #878

With this patch on my machine:

All
  foldMap'Tree Sum: OK (0.31s)
    18.8 ms ± 1.9 ms
  sum:              OK (1.42s)
    18.0 ms ± 761 μs

This patch "changes" the strictness of sum and product, but it has already been different depending on which GHC/base version containers is compiled with. So this change makes it consistent.

[phadej]

Ah, there's already #880