Byte-operator lowering: Factor out unbounded array unpacking

[tautschnig]

unpack_array_vector was large already and will grow in upcoming commits.
This is a refactoring with the only change of a precondition check that
should always have been in place as non-byte aligned array members never
were properly supported.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6476 (b9f4a36) into develop (2a9e3e2) will not change coverage.
The diff coverage is 96.77%.

@@           Coverage Diff            @@
##           develop    #6476   +/-   ##
========================================
  Coverage    76.02%   76.02%           
========================================
  Files         1546     1546           
  Lines       165352   165352           
========================================
  Hits        125711   125711           
  Misses       39641    39641           

Impacted Files	Coverage Δ
src/solvers/lowering/byte_operators.cpp	92.18% <96.77%> (ø)

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