Refactor must_use lint into two parts

Before, the lint did the checking for `must_use` and pretty printing the
types in a special format in one pass, causing quite complex and
untranslatable code.
Now the collection and printing is split in two. That should also make
it easier to translate or extract the type pretty printing in the
future.

Also fixes an integer overflow in the array length pluralization
calculation.

Author: Noratrieb