Add with_capacity constructor

[josecelano]

See: https://docs.rs/skiplist/latest/skiplist/skipmap/struct.SkipMap.html#method.with_capacity

Does it make sense to add a constructor to pre-allocate items in the skip list like in the other implementation.

use skiplist::SkipMap;

let mut skipmap = SkipMap::with_capacity(100);
skipmap.extend((0..100).map(|x| (x, x)));

I'm using the SkipMap, and I wonder if the application would be faster adding items, if we pre-allocated memory.