Fix some warnings

[nightscape]

I'm not sure if some of those warnings were intentional. If so, please let me know and I'll take them out of the PR.

[nightscape]

I addressed your comments and rebased on the latest main. Let's see what the tests say 😉

[nightscape]

@lefou I guess the tests failing on some platforms are unrelated to my changes? If so, this would be good to merge from my side 😃

[lefou]

I think this test error is not unrelated.

 X mill.codesig.CallGraphTests.realistic.3-par-merge-sort 38ms 
  ujson.ParseException: expected whitespace or eof got "*" at index 2817
    ujson.CharParser.die(CharParser.scala:98)
    ujson.CharParser.parse(CharParser.scala:77)
    ujson.StringParser$.transform(StringParser.scala:28)
    ujson.StringParser$.transform(StringParser.scala:27)
    ujson.Readable$fromTransformer.transform(Readable.scala:19)
    upickle.Api.$anonfun$read$1(Api.scala:42)
    upickle.core.TraceVisitor$.withTrace(TraceVisitor.scala:18)
    upickle.Api.read(Api.scala:42)
    upickle.Api.read$(Api.scala:41)
    upickle.default$.read(Api.scala:238)
    mill.codesig.CallGraphTests$.$anonfun$parseJson$8(CallGraphTests.scala:191)
    scala.Option.map(Option.scala:242)
    mill.codesig.CallGraphTests$.parseJson(CallGraphTests.scala:191)
    mill.codesig.CallGraphTests$.testDirectCallGraph(CallGraphTests.scala:112)
    mill.codesig.CallGraphTests$.testExpectedCallGraph(CallGraphTests.scala:97)
    mill.codesig.CallGraphTests$.$anonfun$tests$59(CallGraphTests.scala:13)