Replace internal hashtable with hashbrown RawTable.

Benefits

• Allows only hashing keys once
• Allows a more efficient implementation of the Entry api. Which will always be a single hashtable operation.
  • Previously, building the entry would lookup the hashtable once, and other operations would do another lookup (e.g. OccupiedEntry::remove or VacantEntry::insert).
• Allows dashmap raw api to be safely exposed even with the improvements above, as the shards are raw tables w/o hashers. This will avoid problems like Only hash keys once #259. crates.io use case (iterate the removed shards) will still be possible with rawtables.
• Removes a lot of generic types for the hasher
• Slightly simplifies some code paths as we don't have to fight lifetimes

The first two can be significant performance improvements in some circumstances.

Sadly, this is a breaking change for raw-api users so it'll require a major version bump.

Maybe the third time is the charm 😄