Extract common utils from both adapter files

[markerikson]

This PR:

• Dedupes some of the logic in the unsorted_state_adapter / sorted_state_adapter files by extracting some common utils
• Fixes a leftover stray bit of commented out code in the serializability middleware

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[github-actions]

Size Change: -99 B (0%)

Total Size: 69.7 kB

Filename	Size	Change
dist/redux-toolkit.cjs.development.js	13.8 kB	-39 B (0%)
dist/redux-toolkit.cjs.production.min.js	5.31 kB	+12 B (0%)
dist/redux-toolkit.esm.js	13.7 kB	-43 B (0%)
dist/redux-toolkit.umd.js	25.7 kB	-38 B (0%)
dist/redux-toolkit.umd.min.js	11 kB	+9 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
dist/index.js	149 B	0 B

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[markerikson]

Amusing to note that switching for..of loops to for(let i = 0 loops does drop the size a bit (-94b min+gz). Will skip merging that, though - it'll be taken care of by creating a "modern JS" build.