`let_cxx_string!` uses uninitialized value due to exception safety violations

In affected versions of this crate, `let_cxx_string!` is not exception safe. After creating the `StackString`, if `match $value` panics, the content of `StackString` is not yet initialized, while the drop implementation of `StackString` unconditionally deinitializes the content, leading to use of uninitialized value.

The soundness issue was fixed in version `1.0.195` by moving drop logics to separate drop guard after initializing the `StackString`.