Mark InstrusiveList::new() const to remove need for *Lock in statics (#337)

Reduce amount of code required to initialize and use an IntrusiveList by
marking new const

Author: JamesHuard

embedded-service/src/intrusive_list.rs

@@ -93,7 +93,7 @@ impl Default for IntrusiveList {
 
 impl IntrusiveList {
     /// construct an empty intrusive list
-    pub fn new() -> IntrusiveList {
+    pub const fn new() -> IntrusiveList {
         IntrusiveList { head: Cell::new(None) }
     }