Ensure using expression has a valid type

[travisbrown]

Right now the implementation silently ignores records with extra fields in using, or even ill-typed expressions, but the standard says the expression must have one of two types:

(Δ, parent) × Γ₀ ⊢ requestHeaders ⇒ resolvedRequestHeaders ⊢ Γ₁
ε ⊢ resolvedRequestHeaders : H
H ∈ { List { mapKey : Text, mapValue : Text }, List { header : Text, value : Text } }

[codecov-commenter]

Codecov Report

Merging #295 (e84a798) into main (624a37b) will increase coverage by 0.57%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #295      +/-   ##
==========================================
+ Coverage   58.22%   58.80%   +0.57%     
==========================================
  Files          30       30              
  Lines         936      954      +18     
  Branches       54       61       +7     
==========================================
+ Hits          545      561      +16     
- Misses        391      393       +2     

Impacted Files	Coverage Δ
.../src/main/scala/org/dhallj/imports/ToHeaders.scala	86.95% <90.69%> (+1.24%)	⬆️
...ala/org/dhallj/imports/ResolveImportsVisitor.scala	85.71% <100.00%> (ø)

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