avoid `enum` name

[xuwei-k]

prepare Scala 3

[som-snytt]

psa

scala> def f(@deprecatedName("enum") y: Int) = y
def f(y: Int): Int

scala> f(enum=42)
             ^
       warning: the parameter name enum is deprecated: use y instead
val res1: Int = 42

scala> val enum = 42
           ^
       warning: Wrap `enum` in backticks to use it as an identifier, it will become a keyword in Scala 3. [quickfixable]
val enum: Int = 42