Add vyukov_hash_map

[mpoeter]

Based on https://groups.google.com/forum/?#!topic/lock-free/qCYGGkrwbcA

• [x] trivial key/values
• [x] trivial key/non-trivial value
• [x] non-trivial key/any value
• [x] trivial key/managed_ptr value
• [x] non-trivial key/managed_ptr value
• [x] iterators