compiler: Improve data streaming backend

[FabioLuporini]

Yet another bunch of misc compiler tweaks and generalisations, mostly to better support lazy data streaming in PRO

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 52.97619% with 79 lines in your changes missing coverage. Please review.

Project coverage is 86.69%. Comparing base (e37d6ff) to head (4d08839).

Files	Patch %	Lines
devito/operator/operator.py	43.95%	49 Missing and 2 partials :warning:
tests/test_gpu_common.py	0.00%	7 Missing :warning:
devito/passes/iet/instrument.py	20.00%	4 Missing :warning:
devito/types/basic.py	66.66%	4 Missing :warning:
devito/types/parallel.py	66.66%	3 Missing and 1 partial :warning:
devito/mpi/halo_scheme.py	40.00%	3 Missing :warning:
devito/ir/clusters/algorithms.py	66.66%	1 Missing and 1 partial :warning:
devito/ir/stree/algorithms.py	0.00%	2 Missing :warning:
devito/arch/archinfo.py	66.66%	1 Missing :warning:
devito/types/dense.py	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2390 +/- ## ========================================== - Coverage 86.78% 86.69% -0.09% ========================================== Files 234 234 Lines 44271 44383 +112 Branches 8184 8216 +32 ========================================== + Hits 38419 38477 +58 - Misses 5137 5185 +48 - Partials 715 721 +6 ```

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