Refactor sentence-functions with Proper (using Herodotus in the background)

This refactor was a breeze and it passed all conformance tests on the
first try. This, just like with the word-functions, leads to a massive
simplification and separation of concerns in the code. And as with the
word functions, this fixes some known quirks.

Signed-off-by: Laslo Hunhold <dev@frign.de>