Actually use Hashbrown in the measurements now that it is part of our dependencies.

measurements/src/bin/bipartite.rs

-use std::collections::{HashMap, HashSet};
 use std::env::args_os;
 
+use hashbrown::{HashMap, HashSet};
+
 use measurements::{collect, flower_patches, insects};
 
 fn main() {

measurements/src/bin/observations.rs

-use std::collections::{HashMap, HashSet};
 use std::env::args_os;
 
+use hashbrown::{HashMap, HashSet};
+
 use measurements::{collect, insects};
 use model::{insect::Species as InsectSpecies, observer::Observation};
 

measurements/src/bin/prevalence.rs

-use std::collections::HashMap;
 use std::env::args_os;
 
+use hashbrown::HashMap;
+
 use measurements::{collect, flower_patches, insects};
 use model::{flower_patch::Species as FlowerPatchSpecies, insect::Species as InsectSpecies};