Fix annoying type inference in database context expressions

[agentm]

Example:

 :showexpr relation {spam Maybe Int}{tuple{spam Nothing}}
┌────────────────────────────┐
│spam::Maybe (a::IntAtomType)│
├────────────────────────────┤
│Nothing                     │
└────────────────────────────┘
TutorialD (master): x:=relation{spam Maybe Int}
TutorialD (master): insert x relation{tuple{spam Nothing}}
ERR: TypeConstructorTypeVarsMismatch (fromList ["a"]) (fromList [])

The relvar type used in the insert should be used to resolve the tuple's type.