Add support for accessing/projecting record type fields

[Gabriella439]

No description provided.

[winitzki]

What are the conflicts in the git submodule dhall/dhall-lang? Should this branch get updated with the latest dhall-lang?

[winitzki]

This PR fails with the corner case that I was describing in the discussion here:

⊢ { a : Bool, b : Text }.{}

{=}

⊢ {  }.{}

{=}

If an empty field selection is applied to a literal record type, the result must be {}. In all other cases, the result must be {=}.

[winitzki]

Replace:

if null ks
        then VRecordLit mempty
        else case t of

by

case t of
    VRecord _ | null ks -> VRecord mempty
     _        | null ks -> VRecordLit mempty

In this way, { a : Bool } . { } will be correctly reduced to { }.

[winitzki]

If this branch is too stale, I could prepare another branch with just the code changes (and no changes in the dhall/dhall-lang submodule).