Spec update or mark as out of date

[ss2165]

The specification is now very out of date. In particular relating to:

• Extensions, Packages, Linking
• Polymorphism
• Row variables

It would be wise to update this soon. However, it may be more sensible to do this after settling on a spec for model: #1433 . Since that may start a while, we may want to add a warning to the spec to highlight out of date sections.

Related issues:

• Add Array ops to the spec #1520
• Specify the ordering of different edge kinds in the spec #848
• Spec third party responsibility examples and clarifications #748
• schema: Add an example for the TailLoop container. #663
• Dodgy example of params in the spec #311