Deprecate implicit conversion between signed and unsigned integers

[Walter Bright]

I forgot to mention:

```
int popcount(int x);
```
```
uint y = ...;
popcount(y);   // do you really want that to fail?
```

Let's fix it:

```
int popcount(uint x);
```
```
int y = ...;
popcount(y);   // now this fails
```