Phobos Unittest

[Per Nordlöw]

On Saturday, 4 September 2021 at 13:12:49 UTC, Steven 
Schveighoffer wrote:
> Note that lexing and parsing is extremely quick, and I wouldn't 
> focus on trying to trim this out, you won't get much 
> performance out of that.
>
> -Steve

For the record, a D file containing only `import std;` type 
checks via

```sh
time dmd import_std.d -o-
```

in 0.20s whereas

```sh
time dmd import_std.d -o-
```

in 0.45s on my ThreadRipper.