Fixed keeping INSTANCE field and serializer function for serializable objects in R8 full mode

[shanshin]

Details https://issuetracker.google.com/issues/379996140

Fixes #2861

[sandwwraith]

Why it is not

-if @kotlinx.serialization.Serializable class **
-keepclassmembers class <1> {
    public static <1> INSTANCE;
    kotlinx.serialization.KSerializer serializer(...);
}

as you advised in the original issue?

[shanshin]

as you advised in the original issue?

current rule more strict, serializer function will be kept only if INSTANCE field is present.
If a less strict rule is also acceptable, then it is better to use

-keepclassmembers @kotlinx.serialization.Serializable class ** {
    public static ** INSTANCE;
    kotlinx.serialization.KSerializer serializer(...);
}

[shanshin]

@sandwwraith, I simplify rule, and made it less strict. Now all INSTANCE fields and serializer function will be kept in any class marked as Serializable