Add bindings for QChar

[jnbooth]

A QString is a list of QChars, but cxx-qt-lib doesn't currently have bindings for QChar. That means any time you want to interact with the QString as a list (split, add, remove, etc.), you need to allocate a new QString. This PR adds bindings for QChar so it can be used directly. That also makes it much easier to find out what's inside a QString without needing to convert it to a String.

A QChar is just a standard UTF-16 double byte, so there's no need for FFI calls. In fact, they can be constructed at compile time. That means instead of doing this:

fn split_items(s: &QString) -> QStringList {
    s.split(&QString::from("|"), /* other args */)
}

You can now do this:

const DELIM: QChar = QChar::from_char('|').unwrap();

fn split_items(s: &QString) -> QStringList {
    s.split(DELIM, /* other args */)
}

This is both more efficient (no allocations) and more readable (no "magic" strings).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f41f139) to head (2332336).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1405   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           75        75           
  Lines        13124     13124           
=========================================
  Hits         13124     13124           

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