Special syntax for `std.trace` – similar to assert

Debugging with std.trace can be annoying, because a large part of the code may need to be parenthesized. It would be nicer if the following syntax was possible:

```
trace foo;
[rest of the code]
```

Instead of:

```
std.trace(
foo,
[rest of the code]
)
```

It raises a question whether we have a similar problem with normal functions. I think it's much less of an issue, because trace statements are expected to be added and removed very quickly ad-hoc during debugging, so dealing with formatting and indentation is most painful for it. Also it is used for side-effects and tracing in some sense doesn't care about the rest of the code following it, so this syntax makes sense for it.