Remove outer boxing from Options

[mgeisler]

In #219, the type parameter for Options was relaxed to ?Sized, which means that Options<T> can be a dynamically-sized type by using dyn WordSplitter as the type parameter. This allows code to freely assign both boxed and unboxed WordSplitters to the same variable:

let mut dynamic_opt: Box<Options<dyn WordSplitter>>;
dynamic_opt = Box::new(Options::with_splitter(20, NoHyphenation));
dynamic_opt = Box::new(Options::with_splitter(20, Box::new(NoHyphenation)));

In both cases, dynamic dispatch would be used at runtime. This was called “proper outer boxing” in #219 and #215.

By only boxing the word splitter (so-called “inner boxing”), the outer layer of indirection can be removed:

let mut dynamic_opt: Options<Box<dyn WordSplitter>>;
dynamic_opt = Options::with_splitter(20, Box::new(NoHyphenation));
dynamic_opt = Options::with_splitter(20, Box::new(HyphenSplitter));

This also used dynamic dispatch at runtime.

Static dispatching was also possible by using a fixed type. Trying to change the word splitter type is a compile time error:

let mut static_opt: Options<NoHyphenation>;
static_opt = Options::with_splitter(10, NoHyphenation);
static_opt = Options::with_splitter(20, HyphenSplitter);  // <- error!

In order to add a trait for the WrapAlgorithm enum (see #325), we’re now removing the ?Sized bound on the WordSplitter type parameter. This makes the first block above a compile time error and you must now choose upfront between boxed and unboxed word splitters. If you choose a boxed word splitter (second example), then you get dynamic dispatch
and can freely change the word splitter at runtime. If you choose an unboxed wordsplitter, you get static dispatch and cannot change the word splitter later.

This change seems necessary since we will be adding more type parameters to the Options struct: one for the wrapping algorithm and one for how we find words (splitting by space or by the full Unicode line breaking algorithm).

Since both dynamic and static dispatch remains possible, this change should not fundamentally change the expressive power of the library.