#364, #365: rule id-related issues from Luke

I wrote a change draft that incorporates Luke’s feedback from  Issue #364  (Remove overly restrictive advice on stable, opaque ids) and  Issue #365  (ruleId hierarchical string is incompatible with Semmle rule ids).

For #364, I changed the requirement that rule ids be opaque from  SHALL  to  SHOULD , but left the  SHALL  requirement that the id be stable. I also left the text praising the virtues of opaque ids; actually, the text doesn’t do a great job explaining the rationale, and I’ll take are of that editorially before  the comment period or before Wednesday if I have time. See
this comment  in the issue for more words.

Here's the change draft:

https://github.com/oasis-tcs/sarif-spec/blob/master/Documents/ChangeDrafts/Accepted/sarif-v2.0-issues-364-365-Luke-rule-id.docx

and as always (or at least, as of the last several weeks), the provisional draft includes all changes to date including these:

https://github.com/oasis-tcs/sarif-spec/blob/master/Documents/ProvisionalDrafts/sarif-v2.0-csd02-provisional.docx

Larry