Pushing values in a Treemap should be more restrictive

> Pushes `value` in the treemap only if it is greater than the current maximum value.

Pushing a value in a Treemap should be more restrictive and fit the description of the `Treemap::push` method.

_Originally posted by @Kerollmops in https://github.com/RoaringBitmap/roaring-rs/pull/182#discussion_r800460847_