Allow type variables when spreading record type definitions

[cristianoc]

Allow using type variables when spreading record type definitions.

Check this out for an example of what now works: https://github.com/rescript-lang/rescript-compiler/blob/ad9e0319e59b083f75134739b84b9c2c1dc42218/jscomp/test/record_type_spread.res

[cristianoc]

This example seems to still have some problems:

// tst.res

type t<'a, 'b> = {
  x: result<'a, 'b>,
}

type d = {
  ...t<int, int>,
}

gives

% ./bsc tst.res

  We've found a bug for you!
  tst.res:6:1-8:1

   4 │ }
   5 │ 
   6 │ type d = {
   7 │   ...t<int, int>,
   8 │ }
   9 │ 
  10 │ 

  The implementation tst.res
  does not match the interface (inferred signature):
  Type declarations do not match:
    type d = {x: result<int, int>}
  is not included in
    type d = {x: result<int, int>}
  tst.res:6:1-8:1: Expected declaration
  tst.res:6:1-8:1: Actual declaration
  The types for field x are not equal.

[cristianoc]

This example seems to still have some problems:

// tst.res

type t<'a, 'b> = {
  x: result<'a, 'b>,
}

type d = {
  ...t<int, int>,
}

gives

% ./bsc tst.res

  We've found a bug for you!
  tst.res:6:1-8:1

   4 │ }
   5 │ 
   6 │ type d = {
   7 │   ...t<int, int>,
   8 │ }
   9 │ 
  10 │ 

  The implementation tst.res
  does not match the interface (inferred signature):
  Type declarations do not match:
    type d = {x: result<int, int>}
  is not included in
    type d = {x: result<int, int>}
  tst.res:6:1-8:1: Expected declaration
  tst.res:6:1-8:1: Actual declaration
  The types for field x are not equal.

It fails here (with debug printing in includecore):

          let current_field_consistent =
            let eq = Ctype.equal env true [ld1.ld_type] [ld2.ld_type] in
            if not eq then
            (
              Format.eprintf "ld1.ld_type:\n%a\nld2.ld_type:\n%a\n"
                Printtyp.raw_type_expr ld1.ld_type Printtyp.raw_type_expr ld2.ld_type;
              assert false
            );

Giving:

ld1.ld_type:
{id=488;level=100000000;desc=
              Tconstr(result,
               [{id=916;level=1021;desc=Tconstr(int,[],[])};
                {id=919;level=1021;desc=Tconstr(int,[],[])}],[result])}
ld2.ld_type:

{id=983;level=100000000;desc=
 Tconstr(result,
  [{id=984;level=100000000;desc=Tconstr(int,[],[])};
   {id=985;level=100000000;desc=Tconstr(int,[],[])}],[result])}