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Select the field-handlers based on actual diff of that field present

Filtering only by diff's fields without actual diff values checked
was used since the initial import and release of the framework, and
was only modified in #340 by extending the filtering logic to react
to sub-fields of a bigger structure changed (for example, for a
field "spec.struct.field" when "spec.struct" is added at once).

Yet such an approach is prone to improper detection of actually
changed fields, as documented in #375: when a bigger struct is added
while we expect a non-existent sub-field of it, it is believed to
exist there, as the sub-field matches the bigger struct; in particular,
when an object is created, it is a root-level diff from `None` to actual
content, and all the non-existent fields trigger their field handlers.

By checking if the actual diff of a specific field is not empty, we
can properly detect if that specific field was added, removed, or changed,
be that as a part of a bigger (containing) structure change, or as a change
in any of its (contained) sub-fields.

This comes at a cost of performance loss: the diff reduction is less performant
than a flat iteration over the diff fields: from 20% to 150% (2.5x) slower.
However, the scale of the slowdown is measured in 5-10s per 1'000'000 checks
on MacBook Air 2013, so we can consider it negligible for real operators.

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