[Issue 21169] make checkedint as a drop-in replacement of native int/long

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21169
Issue 21169 depends on issue 21760, which changed state.

Issue 21760 Summary: std.conv.to does not know how to convert a string to a std.experimental.checkedint.Checked!T
https://issues.dlang.org/show_bug.cgi?id=21760

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

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