Fix issue #776

Changes introduced:

• Store keys of let statements to check if any key gets repeated.
• Added 2 new AssignmentError variants:

let x y x = 1 2 3 # AssignmentError::RepeatedKey
let = 1           # AssignmentError::NoKey

• Added FunctionParseError to handle argument (repeated key) and type (wraps TypeError) errors upon header parsing.
• Added unit test for introduced errors.

Closes #776 (closed)