Add @pure and @noSideEffects annotations on extern functions/methods

[jafingerhut]

No description provided.

[jafingerhut]

This is a subset of the changes in #802. It includes only the additions of the @pure and @noSideEffects annotations.

I did add one phrase to qualify the example of a hash function that would be legal to have a @pure annotation -- a hash function that had control-plane writable seed/initialization-vector value should not be annotated @pure.

[jafingerhut]

Since creating this PR, I have made a few more changes to the proposed text. I may have gone overboard with too many words, so please read it and see if my edited version looks reasonable.

[mihaibudiu]

I think you should add a changelog too.

[jafingerhut]

Is the next version number 1.2.1? 1.3.0? Something else? The latest version number mentioned in the revision history right now is 1.2.0, released late 2019.

[mihaibudiu]

We may need to add more stuff there.
Use 1.2.1 and we can fix this later.

[jafingerhut]

Beginnings of a revision history for the next release added. I did not attempt to add all of them, only the one in this PR.

[mihaibudiu]

I think we agreed to merge these changes in the spec.