Closure + Generics are broken
NOTE: https://github.com/mratsim/Arraymancer/issues/258 is still needed and useful; however, after applying it, nimble test fails because of what seems like a regression in Nim (just filed https://github.com/nim-lang/Nim/issues/8432)
while https://github.com/nim-lang/Nim/issues/8432 can hopefully be fixed, I'm filing this issue in case we can have a workaround in the meantime