Reduce allocations in BelongsToAssociation#replace_keys

[casperisfine]

Using the same benchmark as in #51726

replace_keys always allocate a multiple arrays to support composite primary keys, even though most primary keys likely aren't composite.

And even when they are, we can avoid needless allocations by not using Array#zip.

This reduce allocations by 18% (8k) on that benchmark.

Before:

Total allocated: 4.88 MB (44495 objects)
Total retained: 4.16 MB (32043 objects)

allocated memory by file
-----------------------------------
...
 320.00 kB  activerecord/lib/active_record/associations/belongs_to_association.rb

allocated objects by file
-----------------------------------
...
      8000  activerecord/lib/active_record/associations/belongs_to_association.rb

After:

Total allocated: 4.56 MB (36495 objects)
Total retained: 4.16 MB (32041 objects)

NB: belongs_to_association doesn't show up in top files anymore