Adds single cache for type resolvers (avoid duplication)

[eMaringolo]

When you use a FilteredTypeResolver, e.g. with Animal as the root class with Dog and Cat as different types, if you read by looking at Dog and then look by looking at Animal (e.g. session read: Animal), you'll end up with two different instances stored in two different caches (one for Animal and another one for Dog).

This change removes that duplication, and always uses the root descriptor class for the cache of all the inherited instances.

It's old code I'm merging in order to catch up with the main codebase, the discussion comes from here:
https://groups.google.com/g/glorp-group/c/eccmfOergeY/m/OEHmWD-cDgAJ

I've been using it in production since many years now, and all the Glorp test suite pass with this modification in.

[tblanchard]

Looks straight forward. Thanks for the contribution.

[gcotelli]

Some of the integration tests are failing with:
MessageNotUnderstood: receiver of "typeResolver" is nil

[codecov]

Codecov Report

Merging #64 (7b5e681) into master (01bee39) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   77.81%   77.82%   +0.01%     
==========================================
  Files         467      467              
  Lines       48198    48218      +20     
==========================================
+ Hits        37504    37527      +23     
+ Misses      10694    10691       -3     

Impacted Files	Coverage Δ
src/Glorp/CacheManager.class.st	91.42% <100.00%> (+0.45%)	⬆️
src/Glorp/FilteredTypeMapping.class.st	92.10% <100.00%> (+0.32%)	⬆️
src/Glorp/RelationExpression.class.st	86.47% <100.00%> (+0.22%)	⬆️

... and 1 file with indirect coverage changes

[eMaringolo]

Some of the integration tests are failing with: MessageNotUnderstood: receiver of "typeResolver" is nil

It was weird, but I solved it now. The tests pass UndefinedObject as the argument (aClass) and such class doesn't have a descriptor, so no typeResolver either.