Add allAsMap

[sashahe]

If you want to join a map of key and value-futures, each of uniform type, use allAsMap. This
returns a future which completes to a map of all key values of its inputs:

  Map<String, CompletableFuture<String>> futures = new HashMap() {{
    put("key", completedFuture("value"));
  }};
  CompletableFuture<Map<String, String>> joined = CompletableFutures.allAsMap(futures);

[codecov]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #70   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        80        84    +4     
===========================================
  Files              7         7           
  Lines            212       222   +10     
  Branches          14        16    +2     
===========================================
+ Hits             212       222   +10     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/com/spotify/futures/CombinedFutures.java	100.00% <100.00%> (ø)	7.00 <7.00> (ø)
...n/java/com/spotify/futures/CompletableFutures.java	100.00% <100.00%> (ø)	49.00 <8.00> (+4.00)

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[mbruggmann]

The implementation looks good to me. Would be nice to add something to the README?

[sashahe]

I agree, I missed that. I added a description to the README