Fix #458: Implement FutureInsantances for Scala-js

[ghost]

Also introduces exported tests

[codecov-io]

Current coverage is 63.10%

Merging #495 into master will increase coverage by +0.32% as of 7248f98

@@            master   #495   diff @@
=====================================
  Files          154    157     +3
  Stmts         2359   2385    +26
  Branches        66     66       
  Methods          0      0       
=====================================
+ Hit           1481   1505    +24
  Partial          0      0       
- Missed         878    880     +2

Review entire Coverage Diff as of 7248f98

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[ceedubs]

👍

https://gitter.im/non/cats?at=55e1fb611b9798c846ecbc18 has some conversation about this.

[ceedubs]

This has merge conflicts.

[ghost]

this in now waiting on #507, and will need refactoring/re-doing, as in non/alleycats#10 (comment)

[non]

Now that #507 has landed, what parts of this need redoing? As far as I can tell the issue (here at least) is fixed.

[ghost]

@non - What still needs doing is the js version (originally in non/alleycats#10) needs adding.

So as you say, I'll close this and add a new PR (sometime...) for the js parts.