SI-6644 Account for varargs in extmethod forwarder

[retronym]

Which sounded difficult, so instead I offshored the work
to the friendly republic of TreeGen.

Review by @paulp for content, @jsuereth for merge target.

It doesn't seem different from the multitude of prior "value classes + X
are broken" bugs that blocked earlier RCs.

[retronym]

🐱 https://scala-webapps.epfl.ch/jenkins/job/scala-checkin-manual/661/console

[scala-jenkins]

Started jenkins job pr-scala-testsuite-linux-opt at https://scala-webapps.epfl.ch/jenkins/job/pr-scala-testsuite-linux-opt/1610/

[scala-jenkins]

jenkins job pr-scala-testsuite-linux-opt: Failed - https://scala-webapps.epfl.ch/jenkins/job/pr-scala-testsuite-linux-opt/1610/

[scala-jenkins]

Started jenkins job pr-rangepos at https://scala-webapps.epfl.ch/jenkins/job/pr-rangepos/899/

[scala-jenkins]

jenkins job pr-rangepos: Success - https://scala-webapps.epfl.ch/jenkins/job/pr-rangepos/899/

[paulp]

Always a thrill to see someone daring to reuse code. Lgtm.

[paulp]

irony: mkForwarder had zero callers before this commit. So I guess that isn't "reusing code" so much as it is "using code."

[retronym]

Double dose: it was added in that way originally in 2008 (562647a) by the author of the code I updated to use it. Prescient or what!

[jsuereth]

LGTM