Simplify Options by removing generic type parameters

[mgeisler]

This PR removes the three generic type parameters from the Options struct. Before we had

pub struct Options<
    'a,
    WrapAlgo = Box<dyn wrap_algorithms::WrapAlgorithm>,
    WordSep = Box<dyn word_separators::WordSeparator>,
    WordSplit = Box<dyn word_splitters::WordSplitter>,
> {
    // ...
}

and now we simply have

pub struct Options<'a> {
    // ...
}

which is much easier on the eyes. A consequence of this is that the function signatures of wrap, fill, etc are simplified as well. The old functionality is kept intact and there are Custom enum variants which accept a function pointer for quick and easy extensions (though the functions cannot borrow anything — please let me know if this is a concern).

The change here is almost performance neutral with only a 1-2% regression:

% cargo criterion fill_optimal_fit_ascii/4800
String lengths/fill_optimal_fit_ascii/4800
                        time:   [225.48 us 225.50 us 225.51 us]
                        change: [+1.1664% +1.2851% +1.4765%] (p = 0.00 < 0.05)
                        Performance has regressed.

Somewhat surprisingly, this PR is also nearly neutral with regards to the binary size with one configuration taking 4 KB more than before:

Configuration	Binary Size	Delta Before	Delta After
quick-and-dirty implementation	297 KB	— KB	— KB
textwrap without default features	309 KB	12 KB	16 KB
textwrap with smawk	321 KB	24 KB	24 KB
textwrap with unicode-width	321 KB	24 KB	24 KB
textwrap with unicode-linebreak	399 KB	102 KB	102 KB

As soon as we enable the default features, the size difference disappears.