Add support for accessing/projecting record type fields

[Gabriella439]

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[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]

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).