`./koch tools` (including nimble) should be build-able without --nilseqs:on
./koch tools

```
/Users/timothee/git_clone/nim/Nim/dist/nimble/src/nimble.nim(13, 1) template/generic instantiation from here
/Users/timothee/git_clone/nim/Nim/dist/nimble/src/nimblepkg/packageinfo.nim(106, 1) template/generic instantiation from here
/Users/timothee/git_clone/nim/Nim/dist/nimble/src/nimblepkg/packageinfo.nim(110, 25) Error: type mismatch: got <JsonNode, string, nil>
but expected one of:
proc optionalField(obj: JsonNode; name: string; default = ""): string
  first type mismatch at position: 3
  required type: string
  but expression 'nil' is of type: nil

expression: optionalField(obj, name, nil)
    result = optionalField(obj, name, nil)
                          ^
```

/cc @Araq @dom96 we now have a catch 21 situation:
* if I add `--nilseqs:on` in my nim.cfg, nimble doesn't work (eg https://github.com/nitely/nim-regex/issues/18#issuecomment-413724584 `nimble install regex` fails) because it doesn't understand that flag

* if I don't add it, `./koch tools` fails because nimble hasn't been updated yet to support new string semantics (no nil)
