Fix nondeterministic code generation

mockall_derive was internally using HashMap and HashSet and iterating
over their contents.  But by default those types use random internal
state, resulting in nondeterministic output.  Switch them to use
consistent state.

Reported by:	David Tolnay @dtolnay

mockall_derive/src/lib.rs

@@ -12,8 +12,8 @@ use cfg_if::cfg_if;
 use proc_macro2::{Span, TokenStream};
 use quote::{ToTokens, format_ident, quote};
 use std::{
-    collections::{HashMap, HashSet},
     env,
+    hash::BuildHasherDefault
 };
 use syn::{
     *,
@@ -34,6 +34,10 @@ use crate::mock_item::MockItem;
 use crate::mock_item_struct::MockItemStruct;
 use crate::mockable_item::MockableItem;
 
+// Define deterministic aliases for these common types.
+type HashMap<K, V> = std::collections::HashMap<K, V, BuildHasherDefault<std::collections::hash_map::DefaultHasher>>;
+type HashSet<K> = std::collections::HashSet<K, BuildHasherDefault<std::collections::hash_map::DefaultHasher>>;
+
 cfg_if! {
     // proc-macro2's Span::unstable method requires the nightly feature, and it
     // doesn't work in test mode.
@@ -126,7 +130,7 @@ fn deanonymize(literal_type: &mut Type) {
 fn declosurefy(gen: &Generics, args: &Punctuated<FnArg, Token![,]>) ->
     (Generics, Vec<FnArg>, Vec<TokenStream>)
 {
-    let mut hm = HashMap::new();
+    let mut hm = HashMap::default();
 
     let mut save_fn_types = |ident: &Ident, tpb: &TypeParamBound| {
         if let TypeParamBound::Trait(tb) = tpb {