Support conversion of constraint to cube-indexes.

[pp-mo]

Possible approaches for #6617

Code is not tested!
But I've tried to think through what might work, and be useful.

In principle, this should mean that

    r = cube.extract(constraint)

can be replaced with

    inds = constraint.as_indexes(cube)
    r = cube[*inds]

[codecov]

Codecov Report

❌ Patch coverage is 15.38462% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.29%. Comparing base (09feafe) to head (5dccc24).
⚠️ Report is 53 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/_constraints.py	15.38%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6621      +/-   ##
==========================================
+ Coverage   89.95%   90.29%   +0.34%     
==========================================
  Files          91       91              
  Lines       24447    24459      +12     
  Branches     4567     4570       +3     
==========================================
+ Hits        21992    22086      +94     
+ Misses       1686     1602      -84     
- Partials      769      771       +2     

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