It turns out the indexer can only use strings as arguments when
creating a query. Cast `types.CheckID` to a `string` before calling
into `memdb`.
Ideally the indexer would be smart enough to do this at compile-time,
but I need to look into how to do this without reflection and the
runtime package. For the time being statically cast `types.CheckID`
to a `string` at the call sites.
Sean Chittenden