[Beacon Chain] Undeclared identifier ReaderType

[mratsim]

Following our discussion from yesterday I've create a branch that showcase another early symbol resolution issue (nim-lang/Nim#8677)

It is available here: https://github.com/status-im/nim-beacon-chain/tree/serialization_typedesc
The offending proc is: https://github.com/status-im/nim-beacon-chain/blob/8d5ca983/tests/official/fixtures_utils.nim#L113-L126

Running the State tests compile

nim c -r -o:build/test tests/official/test_fixture_state.nim

Note that it will fail at runtime because they do not take into account some changes in 0.6 tests.

But the shuffling tests doesn't

nim c -r -o:build/test tests/official/test_fixture_shuffling.nim

The types are defined here: https://github.com/status-im/nim-beacon-chain/blob/8d5ca983845cfae2628394dbcf7ab8a5b1ce1f75/tests/official/fixtures_utils.nim#L12-L93.

/home/beta/.nimble/pkgs/serialization-0.1.0/serialization.nim(99, 23) Error: undeclared identifier: 'ReaderType'

I tried to use an untyped template as a workaround but it doesn't work.
This is not blocking status-im/nimbus-eth2#263 but I will need to multiply the parseTestFoo proc for the test suite.

Also this might become a blocker for further serialization/deserialization implementations like SOS.

[zah]

This is a classic instance for what I call the Generics "sandwich problem". I've now documented it here:
nim-lang/Nim#11225

The work-around is to export json_serialization in fixtures_utils