Change Collection#slice return type to Collection

[LIQRGV]

This commit should resolve #195

[LIQRGV]

Duplicate of #196

[stof]

This is a BC break, so it cannot be done

[SenseException]

@stof That's why 2.0 is the target branch, but I assume that there are no plans to make a collection return a new collection instance?

[stof]

@SenseException this still looks like a BC break for which it is impossible to provide a smooth migration path, which makes it a very hard BC break for the ecosystem.

[greg0ire]

Closing as per https://github.com/doctrine/collections/pull/264#issuecomment-1072345923