Deprecate implicit conversion between signed and unsigned integers

[Nick Treleaven]

On Sunday, 12 May 2024 at 20:20:10 UTC, Nick Treleaven wrote:
> `cast` can be bug-prone if the original type gets changed. It 
> would be better to have druntime template functions `signed` 
> and `unsigned` to do the casts with IFTI to avoid changing the 
> size of the type.

I forgot, those already exist, at least in Phobos:
https://dlang.org/phobos/std_conv.html#.unsigned