Add analyzer detecting let! _ = ... .

[fkj]

This detects a pattern that makes it easy to accidentally introduce subtle security bugs.

[fkj]

Results of running on the Verify solution

64 occurrences detected, none of which were false positives.
Most occurrences were in test code or in the management API. Most occurrences were validation code which is the target of the analyzer.
Almost of the occurrences look like they could be fixed with minor rewrites.