factor out internal identityFn to utils

[sisp]

Just a minor refactoring. identityFn was defined in several places, so I factored it out into utils/index.ts. One type assertion is necessary in getSnapshot(...) to make it work though.

[codecov]

Codecov Report

Merging #431 (eb4e569) into master (a7bd0c2) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
- Coverage   90.93%   90.93%   -0.01%     
==========================================
  Files         179      179              
  Lines        6388     6387       -1     
  Branches     1157     1157              
==========================================
- Hits         5809     5808       -1     
  Misses        542      542              
  Partials       37       37              

Impacted Files	Coverage Δ
src/snapshot/getSnapshot.ts	95.23% <0.00%> (-0.22%)	⬇️
src/types/primitiveBased/primitives.ts	98.00% <0.00%> (-0.04%)	⬇️
src/types/utility/unchecked.ts	88.88% <0.00%> (ø)
src/utils/index.ts	82.69% <0.00%> (+0.11%)	⬆️

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