- Move the population of the feed's entries into a new function, to make
`BuildFeed` easier to understand/separate concerns/implementation details
- Use `sort+compact` instead of `compact+sort` to remove duplicates
- Change `if !a { a = } if !a {a = }` constructs into `if !a { a = ; if !a {a = }}`.
This reduce the number of comparisons, but also improves a tad the
control-flow readability.
There is no need to detect the format and then the version when both can be
done at the same time.
Add a benchmark as well, on large and small atom and rss files.
Go 1.22 introduced a new [for-range](https://go.dev/ref/spec#For_range)
construct that looks a tad better than the usual `for i := 0; i < N; i++`
construct. I also tool the liberty of replacing some
`for i := 0; i < len(myitemsarray); i++ { … myitemsarray[i] …}`
with `for item := range myitemsarray` when `myitemsarray` contains only pointers.
