Add Miri test to CI and Fix undefined behaviour in a test

[codecov[bot]]

Codecov Report

Merging #67 (f0554ba) into master (50b935b) will decrease coverage by 9.36%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   92.49%   83.12%   -9.37%     
==========================================
  Files           5        5              
  Lines         826      243     -583     
==========================================
- Hits          764      202     -562     
+ Misses         62       41      -21     

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (+4.08%)	:arrow_up:
src/with_alloc.rs	92.64% <0.00%> (-2.89%)	:arrow_down:
src/with_const_generics.rs	91.22% <0.00%> (-2.82%)	:arrow_down:

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[NULLx76]

Also, we might wanna remove the codecov check as it seems to give a lot of false positives