memtable/skiplist: add a purpose-built skiplist

[ajwerner]

Fixes https://github.com/fjall-rs/lsm-tree/issues/95.

Supersedes #98

[marvin-j97]

With the Borrow<Q> trait bound it's not possible to use InternalKeyRef that would allow to not build an InternalKey (which involves a heap allocation). Need to use equivalent::Comparable instead.

See: https://github.com/fjall-rs/lsm-tree/pull/98

[marvin-j97]

Todo:

• Evaluate memory overhead vs crossbeam skiplist
• Evaluate read and write latency vs crossbeam skiplist
• Try in a full system (heavily cached) benchmark (https://github.com/marvin-j97/rust-storage-bench) vs crossbeam skiplist

[marvin-j97]

I've tried rebasing this onto the v3 branch.

Outlook now: we don't need a free list because all insertions will be unique (because of unique sequence number).

[marvin-j97]

Removing the const generic N parameter is actually a bit harder than expected because it defines the layout of Buffer<N>; changing it to static makes pointer derefs fail with misaligned pointer dereference.

[ajwerner]

Makes sense. One day if I find time I'll finish up some of the ideas here.

[marvin-j97]

I deleted the 3.0.0 branch, so this needs to rebased on main now

[marvin-j97]

@ajwerner Can you reopen this PR against main?