Relies on undefined behavior of `char::from_u32_unchecked`

The Windows implementation of this crate relied on the behavior of
std::char::from_u32_unchecked when its safety clause is violated.
Even though this worked with Rust versions up to 1.42 (at least),
that behavior could change with any new Rust version, possibly leading
a security issue.

The flaw was corrected in version 2.0.0.