remove (nearly) useless const for fn

[haraldmaida]

some functions have been declared as const fn although it necessarely could not be used in a const context, or would not make sense.

Re-visited the public API and made const fn non const if there is no good reason for the function to be const.

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.

Project coverage is 89.81%. Comparing base (7e61072) to head (f940761).

Files with missing lines	Patch %	Lines
src/spec/mod.rs	14.28%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #6   +/-   ##
=======================================
  Coverage   89.81%   89.81%           
=======================================
  Files          18       18           
  Lines        1385     1385           
  Branches     1385     1385           
=======================================
  Hits         1244     1244           
  Misses        137      137           
  Partials        4        4           

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