Blocktypes can define their own custom yants types and they will be
checked properly, giving us some actual type safety.

Q: should we use uncheckd imported for the registry (i.e. the
extractor)?
  - would that have any noticeable impact on registry eval?
  - if it's a drv it's already not lazy as soon as .meta is queried
  - when the type is `drv`, the check in yants is
    `drv = typedef "derivation" (x: isAttrs x && x ? "type" && x.type == "derivation");`
    if that's not lazy, then we incur in extra costs for all items that
    otherwise wouldn't have needed to eval a drv
  - practically speaking, the time real sensitive eval is `init`
  - but `init` already incurs in an evaluation for `meta` (display)
  - type checking _could_ have a negative impact when lib.lazyDerivation
    is used, as this only shields `.meta` & `.passthru`, but not `.type`?
    - A: we're lucky, `.type` is lazy in lazyDerivation, see:
      https://github.com/NixOS/nixpkgs/blob/29f41f2a5d31f04e58334376dc08ced7212df197/lib/derivations.nix#L90
  - `.type` or `isAttrs` breaking laziness needs to be validated