Feature request: empty behavior on assert failure

[Dukc]

I think we're lacking a good option for assert failure behaviour 
for released code. Basically, we have two options.

One, crashing on assert failure. This may be a performance 
problem. It is customary for D code to assume the assertions will 
be removed on released code, and thus they are often used with 
little regard for performance.

Two, undefined behaviour. Of course, if an assert has gone boom 
it's not good news no matter what we do. The problem with 
undefined behaviour is that it defeats the purpose of leaving 
bounds checks in released code:
```D
@safe pure int fun(int[] arg)
{ import std.algorithm, std.range;
   assert(arg.length >= 10);
   // Cannot go out of bounds without violating the assertion 
above.
   // Array bounds check may be skipped.
   return arg.takeExactly(10).sum;
}
```

IMO we need an option for the compiler to delete the assertions, 
without being allowed to assume the assertion holds. The compiler 
would still be free to crash on assertion failure, if it deems 
that better for performance, but it could not trigger undefined 
behaviour. In other words, the compiler would treat `assert(x)` 
the same as:
```D
if(!x)
{ int v = void;
   if(v > 0) assert(false);
}
```

I also think this option should be the default, unless 
`-boundscheck=off`.