Refactor command compilation to use function pointers

[dspinellis]

This simplifies command handling by

• replacing the construction and a runtime map lookup with a simple match structure,
• eliminating an extra match lookup on the command argument type,
• eliminating one field in each command specification, and
• making command processing similar for each command.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 164 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (2430672) to head (3fbceac).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/sed/src/compiler.rs	0.00%	164 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #108   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         13      13           
  Lines       2716    2664   -52     
  Branches     224     225    +1     
=====================================
+ Misses      2716    2664   -52     

Flag	Coverage Δ
macos_latest	0.00% <0.00%> (ø)
ubuntu_latest	0.00% <0.00%> (ø)
windows_latest	0.00% <0.00%> (ø)

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[cakebaker]

Nice refactoring, thanks!