disk bucket: refactor storage get/get_mut fns

[brooksprumo]

Problem

1. There is duplicate code in the get/get_mut family of functions on bucket_storage
2. These functions do not have safety checks for size/alignment

Summary of Changes

In commit 1, refactor the impl of the get/get_mut functions
In commit 2, move the functions within the file (no code changes)
In commit 3, remove get_from_parts functions, now that they are unused
In commit 4, fixup the impl from commit 1 😅

[jeffwashington]

lgtm

[codecov]

Codecov Report

Merging #30985 (c188306) into master (38e054f) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #30985   +/-   ##
=======================================
  Coverage    81.5%    81.5%           
=======================================
  Files         728      728           
  Lines      205427   205427           
=======================================
+ Hits       167511   167526   +15     
+ Misses      37916    37901   -15