New behavior with `seq` and object defaults
I really don't know if this is a bug or an improvement but after ccb40024c65ea2d2856818780a5e721cf1db85a7 this snippet behaves differently:
```nim
import std/tables
type
  MyTyp = ref object
    thing = initTable[string,string]()

var t = MyTyp()
t.thing[""] = ""
```

Before the commit this would work, now it's a memory address violation. It seems like the reason is because the table uses a `seq` internally and the seq's data is now a constant. According to the spec object defaults should be constant expressions but on the other hand it is nice to be able to do things like this. Thinking of the default value as a constant is okay too and maybe has some performance implications but I'm rambling at this point.

This doesn't seem to be consistent with other types though:

```nim
type
  MyTyp = ref object
    thing = [1,2,3]

var t = MyTyp()
t.thing[0] = 0
```
same with `string`. Not sure what to make of it.

This is on latest devel.