Add Default macro for non-exhaustive types

[mmynk]

Adding Default macros for a subset of the non_exhaustive structs.

Need this in order to write unit tests when using this library. Without Default macro, creating this struct is extremely tedious. We cannot use struct expression to create them as it results in the below error:

error[E0639]: cannot create non-exhaustive struct using struct expression

It might be worthwhile to consider if the lib should drop #[non_exhaustive] macro altogether because I do not know if the macro is a right choice for a public facing library ^{1 2}.

[cathay4t]

LGTM

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (fb497b3) 64.20% compared to head (a54bea8) 64.20%.

Files	Patch %	Lines
src/tc/qdiscs/fq_codel.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #92   +/-   ##
=======================================
  Coverage   64.20%   64.20%           
=======================================
  Files         140      140           
  Lines        8947     8947           
=======================================
  Hits         5744     5744           
  Misses       3203     3203           

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[cathay4t]

Regarding the non_exhaustive, I am open to suggestions if you have better approach on stabilizing our API when we or kernel add more items or struct or enum.