Use a `lang` JSON property for same-named constructors, instead of naming conventions

[saser]

We (currently) have two presLang constructors -- ModCon and ConCon -- for the two different Con constructors from modLang and conLang, respectively. When converting from presLang to JSON the object either has

{
    "cons": "Con-modLang"
    ...
}

or

{
    "cons": "Con-conLang"
    ...
}

respectively.

Instead, we should give it a lang property, resulting in

{
    "cons": "Con",
    "lang": "modLang"
    ...
}

or

{
    "cons": "Con",
    "lang": "conLang"
    ...
}

respectively.

[hjorthjort]

Will be fixed by API change

[hjorthjort]

Closed by #22