Add fix and unfix functions for parameter fixing and unfixing

[sunxd3]

This PR add the fix and unfix (https://turinglang.org/DynamicPPL.jl/dev/api/#DynamicPPL.fix) functions piloted in DynamicPPL.

These functions are very useful for functions like predict (ref #81).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.02%. Comparing base (b342b3d) to head (3785256).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   84.82%   86.02%   +1.19%     
==========================================
  Files           3        2       -1     
  Lines         145      229      +84     
==========================================
+ Hits          123      197      +74     
- Misses         22       32      +10     

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[yebai]

Thanks @sunxd3, @penelopeysm!