Improve support for immutable states/operators

[goerz]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 57.77778% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 87.3%. Comparing base (ada992f) to head (dcd77e3).
Report is 1 commits behind head on master.

❗ Current head dcd77e3 differs from pull request most recent head 0b35bf9. Consider uploading reports for the commit 0b35bf9 to get more accurate results

Files	Patch %	Lines
src/interfaces/generator.jl	31.9%	15 Missing ⚠️
src/cheby.jl	60.0%	2 Missing ⚠️
src/storage.jl	0.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #76     +/-   ##
========================================
- Coverage    87.4%   87.3%   -0.1%     
========================================
  Files          31      31             
  Lines        2157    2165      +8     
========================================
+ Hits         1885    1889      +4     
- Misses        272     276      +4     

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