Add 'by_ref' api

[LMJW]

Related issue #185

Add "by_ref" version of api for "get_with", "get_with_if", "try_get_with", "optionally_get_with"

[LMJW]

CI failure.

thread 'sync::cache::tests::test_key_lock_used_by_immediate_removal_notifications' panicked at 'assertion failed: `(left == right)`
  left: `Invalidate("a2")`,
 right: `EndNotify("a0", Expired)`: expected[4]', src/sync/cache.rs:3811:13

This might not related to the change

[tatsuya6502]

LGTM. Thank you!

I will run pre-release tests tonight (UTC+0800), and if it goes well, I will release v0.9.5.

Just FYI, I will probably hide (comment out) get_with_if_by_ref for v0.9.5. I am planing to deprecate get_with_if once the entry/entry_by_ref API are ready (v0.10.0?). The same functionality to get_with_if will be provided via entry* APIs. I would like to avoid adding get_with_if_by_ref and then deprecating immediately. I will not remove your code but comment out as we can use it in the entry* APIs.

[LMJW]

Thanks for approving the PR. If I knew it would be deperacated, I would probably not add the new api for the deprecated function. No worries. You can remove it if it is not needed :)

[tatsuya6502]

OK. All tests passed and I published v0.9.5 to crates.io. You will see optionally_get_with_by_ref in the doc.

Thanks for your contribution!

[LMJW]

Nice! Thanks for your review. :)