Rework error structs and implementation

[JSAbrahams]

Summary of current issue

Error implementation could be made more elegant:

• Cause is an unnecessary construct I think.
  Why not just point to another relevant error?
  So we just get a linked list.
• All relevant error constructs should implement error, so they behave like any other error in Rust, making it easier to use the library.
  We can use the thiserror crate

Proposed change

• Remove Cause struct.
  • Change WithCause trait to WithErr<T>, for relevant errors
• Make ParseErr and LexErr derive Error
• Make TypeErr and derive Error
  • Introduce new TypeErr struct which has internal vector of TypeErrs
    Let this struct also derive Error.
    If we add a new method which behaves similar to the new of TypeErr, we can use this in all situations where we have to manually construct a vector of errors.
• Make UnimplementedErr derive Error