Deprecate implicit conversion between signed and unsigned integers

[Kagamin]

FWIW, if you want C# array idiom
```
int count(T)(in T[] a)
{
	debug assert(a.length==cast(int)a.length);
	return cast(int)a.length;
}

long lcount(T)(in T[] a)
{
	debug assert(long(a.length)>=0);
	return long(a.length);
}
```