WIP -- Start refactoring to avoid unsafe outside of table
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Fix as directed by nikomatsakis
Merge remote-tracking branch 'niko/pczarn-hashmap-opt' into hashmap-opt

Conflicts:
	src/libstd/collections/hashmap/map.rs
	src/libstd/collections/hashmap/table.rs
Fix test_resize_policy. Explain
Tests pass
Use SearchResult enum to preserve references and avoid unsafe blocks
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