Use QString::as_slice for String conversions

[jnbooth]

Currently, if you have a qstring: QString and call qstring.to_string(), here is what happens:

1. We allocate a QByteArray.
2. Qt fills the byte array with qstring's UTF-8 data.
3. We allocate a String and copy the data into it.
4. Rust allocates a String of its own.
5. Rust copies the data from our string into its own string.

That's a lot of work and it's unnecessary. All we have to do is get the UTF-16 data of qstring, which is a constant-time operation, and write that into the formatter. String::from(&qstring) is still a bit more efficient than qstring.to_string() with this approach, but the gap is much smaller than before. Arguably small enough that we could just recommend qstring.to_string() for simplicity's sake. I think the only difference is that String::from(&qstring) reserves as much space as it needs up front.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0d38fb2) to head (4a04dbf).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1323   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           75        75           
  Lines        12772     12772           
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  Hits         12772     12772           

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