[rewriter] Create a "why_not_applied" API

[justinchuby]

Call why_not_applied(rule, graph, node: Optional[Node]) and it will print out the reasons why a pattern was not applied.

cc @gramalingam

[gramalingam]

Just adding some thoughts here:

• The reasons should be from the pattern-matching algorithm ... I don't think we want to have a duplicate algorithm for debugging.
• So, the first step would be to make sure that the pattern-matching algorithm logs the reason in some fashion (how it is presented to the user can be done separately, if desired).
• There are places where the algorithm doesn't record any information for efficiency/pragmatism reasons ... eg., for the first node, if the optype doesn't even match (this is a case where user is unlikely to care even for debugging, and efficiency reasons might override).