Commits on Feb 2, 2016

1. Seed HashMaps thread-locally, straight from the OS. …

   This reduces how much rand is required in std, and makes creating
   hash-maps noticably faster. The first invocation of HashMap::new() in a
   thread goes from ~130_000 ns to ~1500 ns (i.e. 86x faster) and later
   invocations go from 10-20 ns to a more consistent 1.8-2.0ns.
   
   The mean for many invocations in a loop, on a single thread, goes from
   ~77ns to ~1.9ns, and the standard deviation drops from 2800ns to
   33ns (the *massive* variance of the old scheme comes from the occasional
   reseeding of the thread rng).
   
   These new numbers are only slightly higher than creating the
   `RandomState` outside the loop and calling `with_state` in it (i.e.
   only measuring the non-random parts of hashmap initialisation): 1.3 and
   18 ns respectively.
   
   This new scheme has the slight downside of being consistent on a single
   thread, so users may unintentionally rely on the order being
   fixed (e.g. two hashmaps with the same contents).
   
   Closes rust-lang#27243.

   

   huonw committed Feb 2, 2016
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