Mark std.file.readText as trusted

`std.file.readText` includes an unsafe cast from `void[]` to `string` but we can verify it can be trusted.

Please instead of marking whole function as `@trusted` do so in minimal possible "grains":

```D
auto result_raw = read(name);
auto result = () @trusted { return cast(S) result_raw; } (); // define lambda and immediately call it
```