Fix type-checking bugs in `let`

[Gabriel439]

Fixes #135

Dhall requires that a let expression of the form:

let x : t = e1 in e2

... should behave the same as the following equivalent lambda expression:

(λ(x : t) → e2) e1

... meaning that if one type-checks then the other should type-check and (vice
versa) if one does not type-check then the other should not type-check. The
only difference should be either:

• the error message changes (i.e. to be more let-specific)
• the order in which errors are checked might differ

I used equational reasoning to discover two corner cases where the two
expressions diverged in their type-checking behavior. This change fixes that
and includes two regression tests to capture the two corner cases to prevent
this problem from recurring