G1: Factor out hash function for G1CardSet

In G1CardSet we use Hotspot's CHT - it requires a few closures to operate, some of them having a get_hash() method that calculates a hash from a given key.

Currently the code inlines the get_hash() method every time (it simply returns the key, but still).

This is inconvenient when doing experiments with different hashes.

Let the get_hash methods call a common method instead.