Conditionally conform Chain2Sequence to MutableCollection

Is there a specific reason why Chain2Sequence does not conform to `MutableCollection` when `Base1` and `Base2` do?

I'm trying to build an API that uses two `some MutableCollection` as parameters and one as return value. I need it to be mutable, because I need to change a specific element in the collection. Internally, the parameters will be chained using `chain(_:_:)` - or at least that's the goal.

I'm happy to provide a PR for this, but I wanted to see if there's a reason against it first.

Locate Chain2Sequence and chain(_:_:), then compare their collection conformances with those of Base1 and Base2. Check Swift’s conditional MutableCollection requirements and existing collection tests; done when the reason for the current behavior and the expected mutability outcome are established.

Written by the indexing model from the issue text.