Bugfix Incorrect Assert in BTree.removeFirst/removeLast

[LeoNavel]

_BTree.removeFirst(_:) and _BTree.removeLast(_:) contains following incorrect assert:

assert(0 <= k && k < self.count, "Can't remove more items from a collection than it contains")

This assertion will cause a crash (in debug mode) when we call one of these functions with the same number of items as the collection contains.

This PR solves this issue by replacing k < self.count with k <= self.count in the assert.

Checklist

• [x] I've read the Contribution Guidelines
• [x] My contributions are licensed under the Swift license.
• [x] I've followed the coding style of the rest of the project.
• [x] I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• [x] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [x] I've updated the documentation if necessary.

[lorentey]

@swift-ci test