Make HoleList public or make Heap::extend accept non contiguous areas

[MarcoCicognani]

Hi, I'm a follower of your work, that is excellent.
I currently develop a Rust OS based on the crates and the blog you wrote, but the user space allocation memory model doesn't follow the classic sbrk-like schema, instead uses a mmap oriented one.
So i gently request you to allow linked_list_allocator accept non contiguous areas in Heap::extend(by: usize) as extensions simply modifying the method to Heap::extend(start_addr: usize, size: usize).
Otherwise you could make public the HoleList module to allow external uses.

best regards Marco Cicognani

[MarcoCicognani]

I know that with an Heap::extend_with_region(start_addr: usize, size: usize) top make no more sense, but in my specific case it doesn't matter

[phil-opp]

Resolved in #47