Safer binary reading (or writing) code

[Joel]

I've got this mixin thing, I think it's less typo-prone. I 
haven't been able to make it show the variable's name, though. 
Also, it should be optional whether it prints anything, (it's not 
hard for me to do that though).

```d
// mixin(jread("width")); -> fread(&width, 1, width.sizeof, 
bfile);
auto jread(string var) => `fread(&`~var~`, 1, `~var~`.sizeof, 
bfile); writeln("read var=", `~var~`);`;
```