Optimize a, c, i text usage

[dspinellis]

Avoid the cost of string copies through the use of a reference counted str.

The benchmark results improve as follows:

access-log-append aci-optimize is 1.07 times faster than rust-f2a2d868

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (f2a2d86) to head (bfac7cf).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/sed/src/compiler.rs	0.00%	2 Missing ⚠️
src/uu/sed/src/processor.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #111   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         13      13           
  Lines       2721    2721           
  Branches     231     231           
=====================================
  Misses      2721    2721           

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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[sylvestre]

nice!