TryPush trait to enable non-reallocating pushes

[antiguru]

Add a TryPush trait that only pushes an item if the region can absorb it without reallocation. This is important for example for pre-sized regions based on some allocation patterns.

The trait offers a try_push function that either returns an index if successful or the item itself if it failed to push it. Additionally, a can_push function takes a reference to the item, and returns a boolean that's true when the region can absorb the item without reallocation.