Create `AsciiStr` from `str` using `const fn`

[PatrickNorton]

Since mem::transmute will be const-stable in 1.56.0, it would be nice if there was a way to construct an AsciiStr (and AsciiString) at compile-time. Since the from_str function is in a trait, and thus cannot be const yet, we would probably need to add a new method for this.