New skip list

[marvin-j97]

Reopening of #131

Stuff to do:

0. Make it compile
1. Make buffer size configurable instead of being a const generic parameter
2. Insertions of duplicates need to be rejected (not panic), to make batch writes with duplicate keys work
3. Evaluate memory overhead vs crossbeam skiplist
4. Evaluate read and write latency vs crossbeam skiplist
5. Try in a full system (heavily cached) benchmark
6. Remove crossbeam-skiplist