Consider renaming `Vec` constructor for `Expr`

[samcowger]

Even though the Haddock promises that this represents an "array literal," I feel that the name Vec is a bit of a red herring. In particular, it suggests to me that it has something to do with std::vec::Vec. I would be much less surprised by a name like Array (though I recognize that conflicts with a constructor for Type).

Just a minor quibble with an otherwise great library!