More stack safety for IndexedReaderWriterStateT & IndexedStateT

[ybasket]

I'm currently adding Sync instances for IndexedReaderWriterStateT to cats-effect (see typelevel/cats-effect#577). Some of the tests there fail as IndexedReaderWriterStateT.flatMap is not stack-safe even when used with a stack-safe monad.

Based on previous work for Kleisli (see #2185), I made flatMap and flatMapF stack-safe by introducing IndexedReaderWriterStateT.shift which implements special behavior if F[_]'s FlatMap instance is also an Applicative.

Furthermore, I applied the usage of AndThen which made IndexedStateT.flatMap stack-safe in #2187 to map as well, allowing for repeated map calls without blowing the stack. The same method unfortunately cannot easily be applied to IndexedReaderWriterStateT as we don't have a Function1 there, so map stays unsafe there.

[codecov-io]

Codecov Report

Merging #2924 into master will decrease coverage by 0.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #2924      +/-   ##
==========================================
- Coverage   94.24%   94.22%   -0.02%     
==========================================
  Files         367      367              
  Lines        7002     6996       -6     
  Branches      195      197       +2     
==========================================
- Hits         6599     6592       -7     
- Misses        403      404       +1

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/IndexedStateT.scala	90% <100%> (ø)	⬆️
...in/scala/cats/data/IndexedReaderWriterStateT.scala	97% <75%> (-1.12%)	⬇️

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