Switch to FxHash for hash maps and such.

This isn't such a massive win as I'd have hoped, but it is consistently faster,
so there's no reason not to.

> ./bindgen-old tests/stylo.hpp --no-rustfmt-bindings > /dev/null 2>&1
6.17s user 0.84s system 98% cpu 7.079 total

> ./target/release/bindgen tests/stylo.hpp --no-rustfmt-bindings > /dev/null 2>
5.92s user 0.87s system 98% cpu 6.866 total

Which isn't _that_ much but it's quite a bit.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 9a7bf897..edaf7afe 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -43,12 +43,13 @@ use proc_macro2::{self, Ident, Span};
 use std;
 use std::borrow::Cow;
 use std::cell::Cell;
-use std::collections::{HashSet, VecDeque};
-use std::collections::hash_map::{Entry, HashMap};
+use std::collections::VecDeque;
+use std::collections::hash_map::Entry;
 use std::fmt::Write;
 use std::iter;
 use std::ops;
 use std::str::FromStr;
+use {HashMap, HashSet};
 
 // Name of type defined in constified enum module
 pub static CONSTIFIED_ENUM_MODULE_REPR_NAME: &'static str = "Type";
@@ -2638,7 +2639,7 @@ impl CodeGenerator for Enum {
         );
 
         // A map where we keep a value -> variant relation.
-        let mut seen_values = HashMap::<_, Ident>::new();
+        let mut seen_values = HashMap::<_, Ident>::default();
         let enum_rust_ty = item.to_rust_ty_or_opaque(ctx, &());
         let is_toplevel = item.is_toplevel(ctx);