Generalize type of Stream.flatMap

[ChrisPenner]

Code review

Stream.flatMap should allow changing the type of the emitted values; flatMap! has the correct type, but flatMap didn't.

P.s. I think I might not have gotten the metadata & licenses quite right; if there are some commands I can run to fix the patch please let me know.

  The changes summarized below are available for you to review, using the following command:

    pull-request.load https://github.com/unisonweb/base:.trunk https://github.com/ChrisPenner/unison-codebase:.prs.base._flatMapFix

  Updates:

     Stream.flatMap : (a ->{e, Stream a} r) -> '{e, Stream a} r -> '{e, Stream a} r
     ↓
     Stream.flatMap : (a ->{e, Stream b} r) -> '{e, Stream a} r -> '{e, Stream b} r
     +  anovstrup.mit_2020                        : License
     +  authors.anovstrup                         : Author
     +  _base.metadata.authors.chrispenner        : Author
     +  _flatMapFix.metadata.licenses.chrispenner : License
     +  _flatMapFix.metadata.authors.chrispenner  : Author

    There were 1 auto-propagated updates.

     patch patch (added 1 updates)

  Added definitions:

     metadata.authors.chrispenner          : Author
     metadata.authors.chrispenner.guid     : GUID
     metadata.copyrightHolders.chrispenner : CopyrightHolder
     metadata.licenses.chrispenner         : License (+1 metadata)

[runarorama]

Merged in base hash #buad16o4j0 🌈🌟