Function places::hash::hash_url_prefix

pub fn hash_url_prefix(spec_prefix: &str, mode: PrefixMode) -> u64

This should be identical to the "real" mozilla::places::HashURL when given a prefix arg. Specifically:

• hash_url_prefix(spec, PrefixMode::Lo) is identical to
• hash_url_prefix(spec, PrefixMode::Hi) is identical to

As with hash_url, it returns a u64, but only the lower 48 bits should ever be set, so casting to e.g. an i64 is lossless.