Add kotlin compiler plugin to generate helpers and schemas

[IlyaGulya]

Current API is a bit cursed.
I'd really like to have an ability to treat Schemas as usual data classes.
At least, when I'm constructing or changing them.
For instance, compiler plugin can generate constructor functions, copy functions, etc.

object Player : Schema<Player>() {
    val Name = "name" let string
    val Surname = "surname" let string
    val Score = "score".mut(i32, default = 0)

    // Can be generated by compiler plugin
    operator fun invoke(
        name: String,
        surname: String,
        score: Int? = null
    ): StructSnapshot<Player> = Player { p ->
        p[Name] = "John"
        p[Surname] = "Galt"
        score?.let { p[Score] = it }
    }
}

// Can be generated by compiler plugin
fun StructSnapshot<Player>.copy(
    name: String? = null,
    surname: String? = null,
    score: Int? = null
): StructSnapshot<Player> = copy { p ->
    name?.let { p[Name] = name }
    surname?.let { p[Surname] = surname }
    score?.let { p[Score] = score }
}