The implicit conversion in `Predef` was made non-implicit in Scala
2.13.4, because of issues on JDK15+. We could explicitly call it,
but the recommended replacement is to use `CharBuffer.wrap`.

In 2.13.4, the pattern match exhaustivity analysis has become
stricter. It remains active in the presence of guards and custom
extractors (including `unapplySeq`). This change causes some new
warnings in our codebase, which this commit addresses.

See scala/scala#9140, which was somewhat
toned down by scala/scala#9299.

Something seems to have changed in Scala 2.13.4 that optimizes
`match`es whose scrutinee is a literal, causing two (similar) tests
to fail. Since the literal scrutinee was not the point of those
tests, we move it to another `def`.