Add an Assert hook to CheckedInt

[wilzbach]

Okay I forgot that I created #5928 a while ago and it seems we now have three options to fix https://issues.dlang.org/show_bug.cgi?id=18024

1. Fix Issue 18024 - checkedint.Warn should be @safe #5928 - remove Warn from Abort
2. Fix Issue 18024 - checkedint.Warn should be @safe (alternative) #6369 - use @trusted Warn
3. this PR - add Assert and alias Abort(T) = Checked!(Checked!(T, Abort), Warn)

All options have their pros/cons, but mostly they don't differ so much, so we should really start to agree on one option..

See also: #6369 and #5928

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[RazvanN7]

This has been fixed by: #7909