Simplify Storage API

[manueldidonna]

Things to do:

• Replace Storage.getMutablePokemon(slot) usages with Storage.getPokemon(slot).toMutablePokemon()
• Use indexed access operator to get and set a pokemon
• Rename deletePokemonto remoteAtand returns the removed pokemon
• Remove Storage.index

interface Storage {
    /**
     * Return a [Pokemon] instance. 
     * @see Pokemon.toMutablePokemon
     *
     * Should throw an [IllegalStateException] if [index] isn't lower than [capacity]
     */
    operator fun get(index: Int): Pokemon
}

interface MutableStorage : Storage {
    /**
     * Replaces the pokemon at the specified [index] in this storage with the specified [pokemon].
     *
     * Should throw an [IllegalStateException] if [index] isn't lower than [capacity]
     */
    operator fun set(index: Int, pokemon: Pokemon)

    /**
     * Removes a pokemon at the specified [index] from the storage.
     * Return the pokemon that has been removed.
     *
     * Should throw an [IllegalStateException] if [index] isn't lower than [capacity]
     */
    fun removeAt(index: Int): Pokemon
}