Add a separate pass to check for missing bindings

[samvv]

Due to issue #47, we need a way to report all unbound identifiers and not just the first one.

One way to solve this would be to never let the type checker inject bindings when it finds that one is missing, but then we also cannot unify the different missing names with each other through a type variable.

The solution left is to create a pass that reports all missing bindings by traversing the AST.