Remove map module from la-arena public API

It’s unlikely that more items will be added to the module,
so it’s simpler for users if `ArenaMap` is re-exported
and the module made private.

crates/hir_def/src/generics.rs

@@ -10,7 +10,7 @@ use hir_expand::{
     name::{name, AsName, Name},
     InFile,
 };
-use la_arena::{map::ArenaMap, Arena};
+use la_arena::{Arena, ArenaMap};
 use syntax::ast::{self, GenericParamsOwner, NameOwner, TypeBoundsOwner};
 
 use crate::{