removed undefined from Core__Nullable

[jquesada2016]

This PR addresses #170. It simply removes external undefined from the Nullable module.

[DZakh]

Undefined is part of nullable. There's Null.t if you dont't need undefined

[jquesada2016]

Wouldn't this be more clear if we had an Undefinable module to seperate null from undefined. I've run across this issue working with JSON.stringifyAny where one of the types is Nullable.t<'a>, and it skipping the field rather than putting null when the value is supposedly null.

[illusionalsagacity]

Wouldn't this be more clear if we had an Undefinable module to seperate null from undefined.

If you want strictly undefined or 'a you probably want an option<'a> instead as that sounds like it'd match your expectations here; and I don't think introducing a breaking change to Core when this matches the behavior of the old Js module would help with the migration path

I've run across this issue working with JSON.stringifyAny where one of the types is Nullable.t<'a>, and it skipping the field rather than putting null when the value is supposedly null.

That sounds like it matches the JavaScript implementation to me:

JSON.stringify({hello: undefined,world: "foo"})// outputs '{"world":"foo"}'