Simplify deflate window

[folkertdev]

this will conflict with #271, but should be simple to fix.

We really don't need this fine-grained initialization: just zero the whole thing out at the start (via the allocator if possible) and get rid of a bunch of bookkeeping.

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zlib-rs/src/deflate.rs	92.85%	1 Missing ⚠️
zlib-rs/src/inflate.rs	80.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
zlib-rs/src/allocate.rs	98.67% <100.00%> (+0.23%)	⬆️
zlib-rs/src/deflate/pending.rs	97.01% <100.00%> (ø)
zlib-rs/src/deflate/window.rs	97.77% <100.00%> (+7.58%)	⬆️
zlib-rs/src/inflate/window.rs	97.95% <100.00%> (+0.38%)	⬆️
zlib-rs/src/read_buf.rs	90.47% <100.00%> (-0.83%)	⬇️
zlib-rs/src/deflate.rs	96.76% <92.85%> (-0.08%)	⬇️
zlib-rs/src/inflate.rs	91.05% <80.00%> (ø)

... and 2 files with indirect coverage changes

[bjorn3]

Would it make sense to return Option<NonNull<T>> instead? Then it would just be a NonNull::new() to get the Option and the return value is half as big.

[bjorn3]

with out without the NonNull change.