Synchronized hashmaps to index your data!

let mut hs = HashSync::new();
hs.insert((1, 2));
hs.insert((1, 3));
hs.insert((3, 4));
let index = hs.index(|&(a, _b)| a);

hs.insert((1, 4));
let rows = index.get_values(&1);
assert_eq!(rows.len(), 3);
assert!(rows.contains(&(1, 2)));
assert!(rows.contains(&(1, 3)));
assert!(rows.contains(&(1, 4)));

Time complexity:

• Index lookups are amortized O(1) (backed by a HashMap).
• Adding new indexes is O(n) where n is the current number of rows.
• Insertions are amortized O(n) where n is the number of indexes.

• Reduce copying (drop Clone requirement on RowT?)
• Drop indexes that are no longer in use