Add flatMapF for StateT

[travisbrown]

I wanted this today and it seems like a reasonable thing to have.

[ceedubs]

👍 thanks, @travisbrown.

While the situation has improved quite a lot after some scala.js changes, we are still seeing some builds (including one for this PR) stall. I'll restart the build, but would you want to comment on #952 as a potential solution?

[codecov-io]

Current coverage is 90.87%

Merging #973 into master will not affect coverage as of a43c17a

@@            master    #973   diff @@
======================================
  Files          181     181       
  Stmts         2159    2160     +1
  Branches        42      42       
  Methods          0       0       
======================================
+ Hit           1962    1963     +1
  Partial          0       0       
  Missed         197     197       

Review entire Coverage Diff as of a43c17a

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[adelbertc]

👍