`std.format` is slow

We're heavily using string concentration via `std.format` and I realized that it dramatically hurts the performance. We had to switch using the old way `'' + ''` and had a huge performance boost. Here is an example:

```
["%s and %s" % ['test', 'test1'] for x in std.range(0, 1000)]
```

The script above is compiled in ~5000ms whereas the following one only takes around 50ms:

```
local test = 'test';
local test1 = 'test1';

[test + 'and' + test1 for x in std.range(0, 1000)]
```

I see that `std.format` is [implemented in Jsonnet](https://github.com/google/jsonnet/blob/master/stdlib/std.jsonnet#L307) but I believe it might be best to implement it as a native function as the logic is complex and it affects the performance dramatically.