Added get! for Dicts

* Refactored setindex! into ht_keyindex2, for use by get! and setindex!
Add docs, do-block function for get!
Add docs, do-block function for get

Also added get, get! for WeakKeyDicts
Merge pull request #5519 from JuliaLang/kms/get!

RFC: Add `get!` for Dicts (addresses #1529)