Support multiple SkipMaps share the same Arena

[al8n]

This feature is a must-have to help upper-level crates implement range-deletions and range-sets.

With this feature, we can have a memtable like this:

struct Memtable {
  skiplist: SkipMap<[u8], [u8]>,
  range_deletions: SkipMap<[u8], ()>,
  range_sets: SkipMap<[u8], [u8]>,
}

In the above struct, skiplist, range_deletions and range_sets, they are separate SkipMap but share the same Arena.

[al8n]

Finished in #48.