Use an object pool for ZopfliHash instances

[Pr0methean]

This avoids a huge allocation (several until #27 is merged) every time the code needs a ZopfliHash instance.

If merging both this and #27, change the declaration of HASH_POOL to

#[cfg(feature = "std")]
pub static HASH_POOL: Lazy<LinearObjectPool<Box<ZopfliHash>>> = Lazy::new(|| LinearObjectPool::new(
    ZopfliHash::new, |hash| hash.reset()));
#[cfg(not(feature = "std"))]
pub static HASH_POOL: Lazy<LinearObjectPool<Box<ZopfliHash>>> = Lazy::new(|| LinearObjectPool::new(
    ZopfliHash::new, ZopfliHash::reset));

[AlexTMjugador]

Thanks! #27 was recently merged, so would you mind rebasing this PR to the latest mainline commit? Also, could you show some benchmark results for comparison?

[andrews05]

For what it's worth, I couldn't measure any performance improvement from this PR 😅.
(Not suggesting it's pointless, just the difference may be negligible)

[Pr0methean]

Rebased.