iterator: Remove ineffective const in base() return type

[amyspark]

Hey @dragonmux,

This fixes a lint in clang-tidy:

Return type 'const iterator_t' is 'const'-qualified at the top level, which may reduce code readability without improving const correctness (fix available) clang-tidy(readability-const-return-type)

It's also picked up by MSVC if warning C5266 is enabled.

[codecov]

Codecov Report

Merging #74 (88c09c1) into main (221b269) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 88c09c1 differs from pull request most recent head 8ec9109. Consider uploading reports for the commit 8ec9109 to get more accurate results

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
+ Coverage   90.38%   90.43%   +0.04%     
==========================================
  Files          47       47              
  Lines        3277     3273       -4     
  Branches      644      640       -4     
==========================================
- Hits         2962     2960       -2     
+ Misses        249      247       -2     
  Partials       66       66              

Files Changed	Coverage Δ
substrate/iterator	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[dragonmux]

Good catch! iterator_t becomes const here anyway due to the function being for const this - please rebase this against main and we'll get this merged.