std.manifestYaml outputs floating point numbers weirdly

This is the same topic as https://github.com/google/jsonnet/issues/558

I've noticed it's fixed for `std.manifestJsonEx`. I'd love to just use `std.manifestJsonEx`, but the problem is that sometimes you need YAML's multi-line string output in order to make things readable so I'm stuck with `std.manifestYaml`.

An example where it makes a difference is the common case of:

```
kubectl diff -f <(jsonnet -y ./foo.jsonnet)
```

where you have something like an environment variable containing a YAML config that also contains potentially multiple levels of nested YAML in multi-line strings. It's pretty readable with `std.manifestYaml` but definitely not at all readable with `std.manifestJsonEx`.