tr: use FnvHashMap instead of default HashMap backed by SipHasher. FnvHashMap is much faster for small inputs like intergers, and we literally have a hashmap with chars as keys

src/tr/Cargo.toml

@@ -10,6 +10,7 @@ path = "tr.rs"
 [dependencies]
 getopts = "*"
 bit-set = "*"
+fnv = "*"
 
 [dependencies.uucore]
 path = "../uucore"

src/tr/tr.rs

@@ -14,6 +14,7 @@
 
 extern crate bit_set;
 extern crate getopts;
+extern crate fnv;
 
 #[macro_use]
 extern crate uucore;
@@ -21,7 +22,7 @@ extern crate uucore;
 use bit_set::BitSet;
 use getopts::Options;
 use std::io::{stdin, stdout, BufRead, BufWriter, Write};
-use std::collections::HashMap;
+use fnv::FnvHashMap;
 
 use expand::ExpandSet;
 
@@ -65,8 +66,7 @@ fn delete(set: ExpandSet, complement: bool) {
 }
 
 fn tr<'a>(set1: ExpandSet<'a>, mut set2: ExpandSet<'a>) {
-    //let mut map = VecMap::new();
-    let mut map = HashMap::new();
+    let mut map = FnvHashMap::default();
     let stdin = stdin();
     let mut locked_stdin = stdin.lock();
     let mut buffered_stdout = BufWriter::new(stdout());