Allow building without std (#1069)

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

cranelift/codegen/src/write.rs

@@ -13,8 +13,8 @@ use crate::ir::{
 use crate::isa::{RegInfo, TargetIsa};
 use crate::packed_option::ReservedValue;
 use crate::value_label::ValueLabelsRanges;
+use crate::HashSet;
 use core::fmt::{self, Write};
-use std::collections::HashSet;
 use std::string::String;
 use std::vec::Vec;