fix(Slab): invalid generation in case of custom config

[loyd]

Hi, I used the following config and found the incorrect calculations of the generation and reference counter. After debugging, I discovered that sometimes Generation::pack is used instead of LifecycleGen::pack, which can be fixed by this PR.

My config:

struct SlabConfig;
impl sharded_slab::Config for SlabConfig {
    const INITIAL_PAGE_SIZE: usize = 32;
    const MAX_PAGES: usize = 15;
    const MAX_THREADS: usize = 256;
    const RESERVED_BITS: usize = 24;
}

The bug cannot be reproduced with the default config, because Generation and LifecycleGen have the same Pack::SHIFT.

It would be nice to remove impl Pack for Generation in favor of different impl Pack for KeyGen and impl Pack for LifecycleGen, but I decided not to do any extra refactoring, which you may not accept.

If the patch is ok for you, I can add a test to prevent regression in the future.

[loyd]

@hawkw, can you check this PR, please?

[loyd]

@hawkw, I've added two integrational tests (I consider them more reliable for preventing regression than the unit ones) that cover 3/5 changed places. I couldn't write a test in half an hour to reveal bugs in fn release2() and fn release() methods (both in integrational and loom tests), so let's merge these ones, it's better than nothing.

[hawkw]

@loyd Okay, that sounds good to me. Thanks for taking the time to add tests at all!

[loyd]

@hawkw, can you release a new release?

[hawkw]

@hawkw, can you release a new release?

@loyd https://crates.io/crates/sharded-slab/0.1.5 :tada: --- sorry to keep you waiting!

[hawkw]

@loyd I've just published a new crates.io release, v0.1.6 to crates.io, which should once again include your changes from this PR. Sorry we had to yank v0.1.5!