compiler: Improve quality of generated code

[FabioLuporini]

This PR, and its sibling in PRO, dramatically improve the quality of the generated code when running production-level kernels. This is mostly evident in PRO, when compression, serialization, mpi, and data streaming are all used in combination in the same Operator. A lot of code is now nicely abstracted away into routines, which significantly decreases the final code size.

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Codecov Report

Attention: 181 lines in your changes are missing coverage. Please review.

Comparison is base (21ef56c) 86.91% compared to head (4dfc228) 86.80%.

Files	Patch %	Lines
tests/test_gpu_common.py	0.00%	74 Missing :warning:
devito/passes/iet/asynchrony.py	7.69%	36 Missing :warning:
devito/passes/iet/engine.py	61.97%	23 Missing and 4 partials :warning:
devito/ir/iet/visitors.py	69.04%	9 Missing and 4 partials :warning:
devito/ir/iet/efunc.py	59.09%	9 Missing :warning:
devito/passes/iet/orchestration.py	0.00%	8 Missing :warning:
devito/passes/iet/languages/openacc.py	44.44%	4 Missing and 1 partial :warning:
devito/core/gpu.py	60.00%	2 Missing :warning:
devito/passes/__init__.py	50.00%	1 Missing and 1 partial :warning:
devito/symbolics/printer.py	33.33%	2 Missing :warning:
... and 3 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2263 +/- ## ========================================== - Coverage 86.91% 86.80% -0.12% ========================================== Files 229 229 Lines 42156 42296 +140 Branches 7787 7830 +43 ========================================== + Hits 36641 36715 +74 - Misses 4865 4921 +56 - Partials 650 660 +10 ```

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