Support multiple keys in bigdiffy

[martinbomio]

Addresses #137.
There were two options that I thought about:

1. Join all keys and to get a single String in *KeyFn functions.
2. Use the list of string representation of the keys as key to the join.

Decided to go with (1) since we will need to join the keys anyways to write the results.

[idreeskhan]

Hmm, what if we change this to a Set[String] instead? My concern is this can produce some weird edge cases. They will probably be rare but it's not really transparent to end user.

e.g.
key1_something, key2 vs key1, something_key2 produces the same aggregate key.

[idreeskhan]

Only suggesting Set since we use that in sampler but open to other ideas

[martinbomio]

wouldn't it be more confusing is the results don't respect the user ordering of the keys?

[idreeskhan]

Maybe some dataset that preserves order instead. Up to you. Doesn't block this PR imo but would be nice to have fixed :)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@9fca065). Click here to learn what that means.
The diff coverage is 61.53%.

@@            Coverage Diff            @@
##             master     #138   +/-   ##
=========================================
  Coverage          ?   70.93%           
=========================================
  Files             ?       37           
  Lines             ?     1421           
  Branches          ?      170           
=========================================
  Hits              ?     1008           
  Misses            ?      413           
  Partials          ?        0

Flag	Coverage Δ
#ratatoolCli	4.48% <0%> (?)
#ratatoolDiffy	28.48% <54.54%> (?)
#ratatoolExamples	19.1% <30.76%> (?)
#ratatoolSampling	63.06% <ø> (?)
#ratatoolScalacheck	83.48% <ø> (?)
#ratatoolShapeless	5.42% <0%> (?)

Impacted Files	Coverage Δ
...om/spotify/ratatool/shapeless/CaseClassDiffy.scala	70.45% <ø> (ø)
...atool/examples/diffy/ProtobufBigDiffyExample.scala	0% <0%> (ø)
...y/ratatool/examples/diffy/PreProcessBigDiffy.scala	100% <100%> (ø)
...in/scala/com/spotify/ratatool/diffy/BigDiffy.scala	53.33% <63.63%> (ø)

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[idreeskhan]

Can you also add a test that runs with multiple keys?
Also would be good to update the docs with the new format.

[idreeskhan]

Sorry, was travelling for a bit. Can you resolve conflicts? and then we can try to get this merged

[martinbomio]

@idreeskhan PTAL, rebased master and added multikey case class

[idreeskhan]

LGTM, if you have a chance could you do some quick and dirty benchmarking to see the performance impact of boxing? If not we can probably just create an open issue for it and revisit if it crops up.

[martinbomio]

@idreeskhan I created #141

[idreeskhan]

👍