Function partition 

pub const fn partition(hash: u64, partitions: usize) -> usize

Returns which of partitions groups hash falls in.

The group comes from the bits just below the fingerprint, which a Table of fewer than 2^40 slots reads for neither the home slot nor the fingerprint. Keys split among tables by group therefore stay spread over every slot and fingerprint of their own table.

Panics

Panics if partitions is not a power of two or exceeds 2^16.