Mutable collections

[BinaryIgor]

We should create MutableCollectionEnvelope. Objects immutability is our core principle, but I think that mutability of collections is something different and it is often the case that we need them (#898).

[0crat]

@llorllale/z please, pay attention to this issue

[wladyan]

@Iprogrammerr I think envelopes must be orthogonal to immutability principle (#947).

[BinaryIgor]

@wladyan I agree. I propose to refactor CollectionEnvelope so that it does not care about immutability as described in #947. We then could add ImmutableCollection decorator that will work in the same manner as CollectionEnvelope is right now. I am willing to do that. @llorllale WDYT?

[llorllale]

@Iprogrammerr this is a duplicate of #947

[0crat]

Job gh:yegor256/cactoos#1008 is not assigned, can't get performer

[0crat]

This job is not in scope