BTreeMap::new_in should be const

[Kritzefitz]

The title basically already says it all. alloc::collections::btree_map::BTreeMap::new_in is currently not marked as const, even though there doesn't seem to be a good reason why it shouldn't be.

[Amanieu]

Sounds good! Feel free to submit a PR and assign it to me for review.

[Kritzefitz]

If I'm not mistaken, the merged PR fixes this issue.