Assign FIXMEs to me and remove obsolete ones

Also fixed capitalization of documentation

library/alloc/tests/lib.rs

@@ -25,8 +25,6 @@
 #![feature(string_remove_matches)]
 #![feature(const_btree_new)]
 #![feature(const_trait_impl)]
-// FIXME remove this when const_trait_impl is not incomplete anymore
-#![allow(incomplete_features)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};