Override assert handler

[ryuukk_]

```
import core.stdc.stdio;

void main()
{
     assert(false, "no");
}

extern(C) void _d_assert_msg (string msg, string file, uint line)
{
     printf("assert failed: %.*s:%u %.*s\n", cast(int)file.length, 
file.ptr, line, cast(int)msg.length, msg.ptr);
}
```

```
assert failed: onlineapp.d:4 no
```


This works

That's funny, because i was thinking about that just right now, 
and how the default message of druntime sucks ass

```
core.exception.AssertError at onlineapp.d(5): no
----------------
??:? _d_assert_msg [0x5625e324e580]
./onlineapp.d:5 _Dmain [0x5625e324e4e4]

```


assert message is obfuscated and surrounded with unpleasant 
characters to read, it should be clear by default, i want to send 
a PR but i can't be bothered to read druntime codebase, a waste 
of time