Allow building without std (#1069)

Closes https://github.com/CraneStation/cranelift/issues/1067

cranelift/codegen/src/ir/constant.rs

@@ -9,8 +9,9 @@
 //! - bucketing constants by size.
 
 use crate::ir::Constant;
+use crate::HashMap;
 use cranelift_entity::EntityRef;
-use std::collections::{BTreeMap, HashMap};
+use std::collections::BTreeMap;
 use std::vec::Vec;
 
 /// This type describes the actual constant data.

cranelift/codegen/src/ir/dfg.rs

@@ -13,12 +13,12 @@ use crate::ir::{
 use crate::isa::TargetIsa;
 use crate::packed_option::ReservedValue;
 use crate::write::write_operands;
+use crate::HashMap;
 use core::fmt;
 use core::iter;
 use core::mem;
 use core::ops::{Index, IndexMut};
 use core::u16;
-use std::collections::HashMap;
 use std::vec::Vec;
 
 /// A data flow graph defines all instructions and extended basic blocks in a function as well as