compiler: Faster compilation time

[FabioLuporini]

the key ingredients are:

• A lot of "avoided, useless reconstructions" -- across all layers (equations, Clusters, IET)
• More caching (which needed the above), again at all levels
• More memoization
• New loop fusion heuristics to avoid useless and expensive analysis

Bonus: a lot of, IMHO, good refactoring. And several new tests.

Compilation time improvements vary between 1.2x and 5x depending on the complexity of the Operator

[codecov]

Codecov Report

❌ Patch coverage is 93.63057% with 70 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.92%. Comparing base (91d8596) to head (392fc8a).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
devito/passes/clusters/fusion.py	88.82%	16 Missing and 4 partials ⚠️
devito/ir/support/guards.py	27.27%	8 Missing ⚠️
devito/symbolics/manipulation.py	80.95%	4 Missing and 4 partials ⚠️
devito/ir/clusters/cluster.py	94.31%	5 Missing ⚠️
devito/passes/iet/engine.py	87.80%	3 Missing and 2 partials ⚠️
devito/tools/memoization.py	93.58%	2 Missing and 3 partials ⚠️
devito/ir/iet/visitors.py	93.10%	4 Missing ⚠️
devito/tools/utils.py	71.42%	3 Missing and 1 partial ⚠️
devito/ir/support/basic.py	96.80%	1 Missing and 2 partials ⚠️
devito/ir/iet/nodes.py	94.44%	1 Missing and 1 partial ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2928      +/-   ##
==========================================
- Coverage   83.35%   82.92%   -0.43%     
==========================================
  Files         248      249       +1     
  Lines       51734    52402     +668     
  Branches     4463     4532      +69     
==========================================
+ Hits        43122    43456     +334     
- Misses       7859     8162     +303     
- Partials      753      784      +31     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	69.00% <88.11%> (+0.29%)	⬆️
pytest-gpu-gcc-	78.27% <91.81%> (+0.23%)	⬆️
pytest-gpu-icx-	78.18% <91.71%> (+0.22%)	⬆️
pytest-gpu-nvc-nvidiaX	?

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