pub const fn new() -> BTreeMap<K, V>
    where
        K: Ord,
    {
        BTreeMap { root: None, length: 0 }
    }
Returns the first key-value pair in the map.
The key in this pair is the minimum key in the map.

# Examples

Basic usage:

```
#![feature(map_first_last)]
use std::collections::BTreeMap;

let mut map = BTreeMap::new();
assert_eq!(map.first_key_value(), None);
map.insert(1, "b");
map.insert(2, "a");
assert_eq!(map.first_key_value(), Some
benchmarks/haystacks/code/rust-library.rs