Allow @Serializer annotation on type parameter

[NiematojakTomasz]

I was trying to workaround #614 by providing custom serializer to Nothing type. Seems it's currently impossible to provide custom serializer for type passed as generic type parameter.

Example:

@Serializable
sealed class CommandExecutionResult<out Result> {
    @Serializable
    class Success<out Result>(
        val result: Result
    ) : CommandExecutionResult<Result>() {
        override fun <T> map(function: (Result) -> T): Success<T> {
            return Success(function(result))
        }
    }

    @Serializable
    class Failure(
        val reason: FailureReason
    ) : CommandExecutionResult<@Serializer(NothingSerializer::class) Nothing>() {
        override fun <T> map(function: (Nothing) -> T): Failure = this
    }
}

[sandwwraith]

Use @Serializable instead of @Serializer