bump-allocator: document & poison (paritytech#7277)

* bump-allocator: refine comments

* Rename EMPTY_MARKER to Nil

It's a misnomer since it doesn't actually denote an empty list as it might suggest but rather the end of a list. So I borrowed Nil from Cons/Nil.I could've gone with Null, especially considering that the variant name is already named `Null`, but I preferred `Nil` because the NIL_MARKER is not 0 as `null` may suggest, but rather 0xFFFFFFFF.

* Rename N to N_ORDERS; docs;

Supply a, hopefully, more readable comment as well.

* allocator poisoning: document and enforce.

* Review: Fix line wrapping for a call

Co-authored-by: Bastian Köcher <[email protected]>

* add a test on oom

* Update primitives/allocator/src/freeing_bump.rs

* Make ui tests happy

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>