Fix mem alignment

[pjtatlow]

This PR resolves the issues that were surfaced by the changes in rust 1.70, where misaligned pointer dereferences now cause panics. The issues were two-fold:

1. When allocating memory using bumpalo we weren't specifying the alignment, so adding the alignment to the layout fixed that.
2. When getting a BucketMeta from the memory mapped data, it's likely that data won't be aligned properly. The fix for this one is to copy the data (16 bytes) to a buffer and then cast it to a *BucketMeta, which we then dereference and return.

Fixes #26

[codecov]

Codecov Report

Patch coverage: 88.23% and project coverage change: -0.10 ⚠️

Comparison is base (00741c7) 94.50% compared to head (5a8a6f3) 94.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   94.50%   94.40%   -0.10%     
==========================================
  Files          21       21              
  Lines        3782     3788       +6     
==========================================
+ Hits         3574     3576       +2     
- Misses        208      212       +4     

Impacted Files	Coverage Δ
src/errors.rs	75.40% <0.00%> (-5.30%)	⬇️
src/bucket.rs	97.24% <100.00%> (+0.02%)	⬆️
src/freelist.rs	97.83% <100.00%> (-0.04%)	⬇️
src/node.rs	95.82% <100.00%> (-0.25%)	⬇️
src/tx.rs	88.41% <100.00%> (-0.04%)	⬇️

... and 2 files with indirect coverage changes

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