Refactor Duration#inspect

In preparing https://github.com/rails/rails/pull/11855, it took me a
minute to understand what was going on due to naming (parts refers first
to an attr_accessor, then to a local, and is then reassigned), but also
because the iterator conditionally builds nulls and then removes them.

I refactored to something much more functional-looking that I find
easier to read, but you may or may not. If you do, great! Enjoy! If
not, oh well, I tried. Can't win 'em all :)

Rationale:

* no name conflict between local var and attr_accessor
* no reassignment of local var
* algorithm spelled out in steps
* unused items in initial list filtered out early
* empty-list case handled early instead of reassigning local var
* no duplication of formatting strings ("0 seconds")

Benchmarks (after PR #11855 merged):

```
10000.times do
  1.second.inspect
end

original #inspect
   0.350000   0.000000   0.350000 (  0.354709)
   0.330000   0.000000   0.330000 (  0.331885)
   0.330000   0.000000   0.330000 (  0.334441)

refactored #inspect
   0.340000   0.000000   0.340000 (  0.340080)
   0.340000   0.010000   0.350000 (  0.345069)
   0.330000   0.000000   0.330000 (  0.335873)

10000.times do
  (1.day + 1.month + 2.minutes + 1.day).inspect
end

original #inspect
   0.400000   0.000000   0.400000 (  0.403027)
   0.400000   0.000000   0.400000 (  0.403781)
   0.390000   0.000000   0.390000 (  0.387596)

refactored #inspect
   0.400000   0.010000   0.410000 (  0.399792)
   0.400000   0.000000   0.400000 (  0.404145)
   0.400000   0.000000   0.400000 (  0.403820)
```