Generics: getTypeInst can return an unusable symbol

[mratsim]

getTypeInst can return a symbol that is not the type we expect but an nnkSym that we passed like T or E and cannot be manipulated.

See status-im/nimbus-eth2#2219 writeup 9

9. Unfixed, the recordFields macro in stew/shims doesn't save instantiated generics and in some cases return the abstract generic type instead of the instantiated one. There is an attempt to monkey patch around that in nim-serialization 82cdd5c.
   It mostly works, except when getTypeInst decides to return a dumb symbol instead of the type we actually want for unknown reason.

Another instance of nim-lang/RFCs#44