Add special syntax for lists on the Halicore side

[dagit]

[brianhuffman]

e342a0037ae221121457 adds a definition for the list datatype, and also defines type-level syntax "[]" => "List" and "[a]" => "List a".

halicore_data List a = Nil | Cons "a" "List a"

However, I haven't implemented expression-level syntax for Nil and Cons yet, because I'm not sure what to do with the type arguments, which are implicit in Haskell but explicit in Core.