Deprecate implicit conversion between signed and unsigned integers

[Walter Bright]

On 2/7/2025 1:04 PM, DLearner wrote:
> Or, maintaining size_t, make first index of an array 1 not 0, and return 0 if 
> not found.
> Like malloc.
> 
> First array index is 1 also eliminates a fruitful source of off-by-one errors.

That's FORTRAN style. It would break about every piece of D code.