[meta] Add a MapWithDefault trait;

This traits augments HashMap so they have a `get_or_default` method that
can be used to avoid boilerplate around usage of `entry.or_default`.

cranelift/codegen/meta/src/lib.rs

@@ -12,6 +12,7 @@ mod gen_settings;
 mod gen_types;
 
 mod constant_hash;
+mod default_map;
 mod shared;
 mod unique_table;