A concurrent insert only hash map.
This crate implements a “memo map” which is in many ways similar to a HashMap with some crucial differences:
```rust use memo_map::MemoMap;
let memo = MemoMap::new(); let one = memo.getorinsert(&1, || "one".tostring()); let one2 = memo.getorinsert(&1, || "not one".tostring()); asserteq!(one, "one"); asserteq!(one2, "one"); ```