Convert to and from AsciiString and Box<AsciiStr>

[A248]

A String is usually preferred to a Box<str>. However, there is one good reason to prefer the Boxed variant to the growable String, which is that String requires more memory. See https://users.rust-lang.org/t/use-case-for-box-str-and-string/8295/4

Similarly, Box<AsciiStr> uses less memory than AsciiString, so it would be helpful if there were ways to convert a AsciiString to a Box<AsciiStr>, just as it is possible to do so for String and Box<str>.

On my machine, Box<AsciiStr> uses 16 bytes compared to 24 bytes for AsciiString.

[tormol]

Implemented in #87 and now released in version 1.1.0.