Improve AndThen use of Single

[johnynek]

follow up to #3527

Here we still do O(1) work on andThen and compose, but we check improve in two ways:

1. we check to see if we have a Concat(_, Single(_, _)) that we can extend. Otherwise the first Concat will break the andThen chaining up to fusionMaxStackDepth.
2. we check to see if the andThen/compose arguments are AndThen instances early. This is a safety fix, otherwise Concat(_, ).andThen(Concat(, _)) puts the right concat inside a single apply call in the loop, which could blow the stack since inside there could also do the same.

Note, in the fallback we were already testing for _: AndThen, here we inline that test to always happen early so we can leverage that information.

[codecov-commenter]

Codecov Report

Merging #3560 into master will decrease coverage by 0.05%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master    #3560      +/-   ##
==========================================
- Coverage   91.29%   91.23%   -0.06%     
==========================================
  Files         386      386              
  Lines        8577     8589      +12     
  Branches      240      252      +12     
==========================================
+ Hits         7830     7836       +6     
- Misses        747      753       +6     

[travisbrown]

This looks reasonable to me.

[PeterPerhac]