change Fn to FnMut in add_periodic_task_to_core

[JakkuSakura]

Description

Fixes # (issue) - if applicable

Type of change

• [ ] Bug fix
• [x] New feature
• [ ] New protocol
• [ ] Breaking change
• [ ] Documentation update

Checklist

• [x] Associated tests
• [x] Associated documentation

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #134 (c6a48e1) into master (44771ea) will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
- Coverage   68.40%   68.38%   -0.02%     
==========================================
  Files          66       66              
  Lines        5972     5972              
==========================================
- Hits         4085     4084       -1     
- Misses       1887     1888       +1     

Impacted Files	Coverage Δ
core/src/runtime/mod.rs	0.00% <ø> (ø)
core/src/testils/proptest/strategy.rs	82.24% <0.00%> (-0.47%)	:arrow_down:

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