Improve performance of pragma-region attach/detach by using transformers

[mlange05]

A subtle but significant performance bug meant that attaching/detaching pragma-regions was becoming ver expensive for large code regions, eg. when finding marked regions to remove (PR #276). This PR fixes this by re-implementing the matching and resolution of PragmaRegion objects with custom in-place transformers that perform the matching at tuple-level.

One important consideration here is that we now only match pragma-pairs at tuple level, and thus have an implicit check for misplaced !$loki end <marker> pragmas, that will now trigger warnings. We also need to be very careful about tuple-insertion when resolving the regions objects during detach, as we do not want nested tuples in our IR tree, but cannot use flatten on tuples generically. This is marked via comments in the respective places.

And finally, some anecdotal speed-up evidence using an experimental ec-physics inlined control flow routine:
Before

(loki_env) $ loki-ecphys-gen.py inline --source ./input  --build .
[Loki::Sourcefile] Constructed from input/ec_phys_drv.F90 in 0.89s
[Loki::Sourcefile] Constructed from input/ec_phys.F90 in 1.37s
[Loki::Sourcefile] Constructed from input/callpar.F90 in 2.56s
[Loki] Inlined EC_PHYS in 2.78s
[Loki] Inlined CALLPAR in 4.76s
[Loki] Remove marked regions in 136.73s   <= this uses `with pragma_regions_attached`

after:

(loki_env) $ loki-ecphys-gen.py inline --source ./input  --build .
[Loki::Sourcefile] Constructed from input/ec_phys_drv.F90 in 0.88s
[Loki::Sourcefile] Constructed from input/ec_phys.F90 in 1.48s
[Loki::Sourcefile] Constructed from input/callpar.F90 in 2.53s
[Loki::EC-Physics] Inlined EC_PHYS in 2.78s
[Loki::EC-Physics] Inlined CALLPAR in 4.90s
[Loki::EC-Physics] Remove marked regions in 2.62s

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/286/index.html

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.87%. Comparing base (a594b44) to head (024018b).

Files	Patch %	Lines
loki/pragma_utils.py	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #286      +/-   ##
==========================================
- Coverage   92.88%   92.87%   -0.01%     
==========================================
  Files         102      102              
  Lines       18253    18269      +16     
==========================================
+ Hits        16954    16968      +14     
- Misses       1299     1301       +2     

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	92.84% <94.11%> (-0.01%)	⬇️
transformations	92.22% <ø> (ø)

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[reuterbal]

Many thanks! Another long-standing utility done right 😄
Implementation looks good to me and the speed-up is extremely impressive!