[refactor] Add a powerful repr() function for debugging

[vkarak]

This function replaces the older debug.repr() and uses util.ppretty() for nicely printing the object attributes. util.ppretty() is also extended in order to recursively call the new repr() function. Objects are printed only once by this new repr() avoiding infinite recursions.

Finally, we use that function to print the whole ReFrame runtime state in case #1369 appears.

This function does not take into account the __slots__ for the moment. In cases, where an object does not have a __dict__ attribute, the builtin repr() will be used.

An example output for a list of objects with self references is the following (see also the corresponding unit test):

[
    _Y({
        'x': 1,
        'y': 'foo',
        'z': _Y(...)@0x102463550,
        'a': None
    })@0x102463550,
    _Y({
        'x': 2,
        'y': 'foo',
        'z': _Y(...)@0x102463510,
        'a': _X({
            '_a': False
        })@0x102463290
    })@0x102463510
]

[vkarak]

Check comment in issue #1369 as a proof that this PR works wonderfully :-) It served its purpose already!

[ekouts]

lgtm

[codecov-commenter]

Codecov Report

Merging #1410 into master will increase coverage by 0.00%.
The diff coverage is 93.33%.

@@           Coverage Diff           @@
##           master    #1410   +/-   ##
=======================================
  Coverage   91.79%   91.79%           
=======================================
  Files          83       82    -1     
  Lines       12743    12712   -31     
=======================================
- Hits        11697    11669   -28     
+ Misses       1046     1043    -3     

Impacted Files	Coverage Δ
reframe/core/config.py	88.62% <ø> (-0.04%)	⬇️
reframe/core/logging.py	82.91% <ø> (+0.35%)	⬆️
reframe/frontend/executors/__init__.py	99.02% <ø> (+0.62%)	⬆️
reframe/frontend/loader.py	91.39% <ø> (+0.77%)	⬆️
reframe/frontend/statistics.py	91.60% <ø> (+0.51%)	⬆️
unittests/test_policies.py	98.66% <25.00%> (-0.80%)	⬇️
reframe/utility/__init__.py	92.56% <100.00%> (+0.52%)	⬆️
unittests/test_utility.py	99.71% <100.00%> (-0.01%)	⬇️

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