Generics regression: getTypeInst causes issue with generic constraints

[mratsim]

In Constantine the following type used to be deserializable.

https://github.com/mratsim/constantine/blob/023e690/tests/t_ec_sage_template.nim#L95-L100

type
  TestVector*[EC: ECP_ShortW_Aff] = object
    id: int
    P: EC
    scalar: matchingScalar(EC)
    Q: EC

It now breaks mratsim/constantine#113

The stacktrace points to getTypeInst

nim-serialization/serialization/object_serialization.nim

Lines 98 to 104 in 4e2ffe3

	for field in recordFields(typeImpl):
	if field.readPragma("dontSerialize") != nil:
	continue
	let
	fieldType = field.typ
	FieldTypeSym = getTypeInst(fieldType)

/home/beta/Programming/Nim/constantine/tests/t_ec_sage_bls12_377.nim(18, 28) template/generic instantiation of `run_scalar_mul_test_vs_sage` from here
/home/beta/Programming/Nim/constantine/tests/t_ec_sage_template.nim(192, 26) template/generic instantiation of `loadVectors` from here
/home/beta/Programming/Nim/constantine/tests/t_ec_sage_template.nim(178, 16) template/generic instantiation of `loadFile` from here
/home/beta/.nimble/pkgs/serialization-0.1.0/serialization.nim(96, 11) template/generic instantiation of `readValue` from here
/home/beta/.nimble/pkgs/serialization-0.1.0/serialization.nim(46, 9) template/generic instantiation of `readValue` from here
/home/beta/.nimble/pkgs/json_serialization-0.1.0/json_serialization/reader.nim(497, 11) template/generic instantiation of `totalSerializedFields` from here
/home/beta/.nimble/pkgs/serialization-0.1.0/serialization/object_serialization.nim(179, 36) template/generic instantiation of `totalSerializedFieldsImpl` from here
/home/beta/.nimble/pkgs/serialization-0.1.0/serialization/object_serialization.nim(176, 26) template/generic instantiation of `enumAllSerializedFields` from here
/home/beta/.nimble/pkgs/serialization-0.1.0/serialization/object_serialization.nim(153, 32) template/generic instantiation of `enumAllSerializedFieldsImpl` from here
/home/beta/.nimble/pkgs/serialization-0.1.0/serialization/object_serialization.nim(104, 33) Error: cannot instantiate TestVector
got: <EC: ECP_ShortW_Aff>
but expected: <EC: ECP_ShortW_Aff>

Another instance of nim-lang/RFCs#44