Improve VecCache under parallel frontend

This replaces the single Vec allocation with a series of progressively
larger buckets. With the cfg for parallel enabled but with -Zthreads=1,
this looks like a slight regression in i-count and cycle counts (<0.1%).

With the parallel frontend at -Zthreads=4, this is an improvement (-5%
wall-time from 5.788 to 5.4688 on libcore) than our current Lock-based
approach, likely due to reducing the bouncing of the cache line holding
the lock. At -Zthreads=32 it's a huge improvement (-46%: 8.829 -> 4.7319
seconds).

compiler/rustc_data_structures/src/lib.rs

@@ -22,6 +22,7 @@
 #![feature(auto_traits)]
 #![feature(cfg_match)]
 #![feature(core_intrinsics)]
+#![feature(dropck_eyepatch)]
 #![feature(extend_one)]
 #![feature(file_buffered)]
 #![feature(hash_raw_entry)]
@@ -79,6 +80,7 @@ pub mod thinvec;
 pub mod transitive_relation;
 pub mod unhash;
 pub mod unord;
+pub mod vec_cache;
 pub mod work_queue;
 
 mod atomic_ref;