Consider arrayvec::ArrayString's trick for making the inline size generic

From https://www.reddit.com/r/rust/comments/3v2uzv/inlinable_string_a_dropin_replacement_for/cxjwhvx:

> Nice. There's a more generic version for the inline part called ArrayString in crate arrayvec (contributed by @tbu- !); it's generic over a set of backing sizes. Feel free to copy the array-parametrization approach.

My original fear was that this would break `StringExt` from being object safe, but I see now that the `InlineString` impl(s) and not the `StringExt` trait would use this so there is nothing to worry about.