Pull `merged` method up from HashMap to Map

[lrytz]

HashMap has a merged method that would be useful to have on any (immutable?) Map (discussed at scala/scala#8134 (comment)).

[nogurenn]

Is this something I could do? How should I start?

[lrytz]

We don't have a 2.14.x branch yet, so cannot merge a PR due to binary compatibility, but feel free to open a PR anyways (there are a frew others with 2.14.0-RC1 milestone already).