Support for labeled tuples

[andreypopp]

No description provided.

[andreypopp]

@anmonteiro I've bumped ppxlib but nix insists on older 0.37 version... what did I do wrong?

[Khady]

probably worth adding a test for a tuple that doesn't have all of its members labelled? Like (x:int * string)

[pedrobslisboa]

@andreypopp I pushed a commit into this branch to support the partially labeled

[pedrobslisboa]

@andreypopp, with the partially labeled, we have the json on non-label as the index of the tuple. Like this:

type t = (x:int * string)[@@deriving json]

{
  "x": 100,
  "1": "Foo"
}