object constructor generation field reordering in sem breaks order of evaluation

i.e.

```nim
type Foo = object
  a, b: int

let x = Foo(
  b: foo()
  a: bar())
```

gets rewritten to:

```nim
Foo(
  a: bar()
  b: foo())
```

in an attempt to set every field. It should probably use the order of the given fields, then set the defaults of the remaining fields (or do this first).