try to tidy up uri-as-string vs. uri-as-object

try to tidy up uri-as-string vs. uri-as-object
I get confused about whether a given argument or return value is a uri-as-string or uri-as-object.  This patch adds a lot of assertions that it is one or the other, and also changes CheckerResults to take objects not strings.
In the future, I hope that we generally use Python objects except when importing into or exporting from the Python interpreter e.g. over the wire, the UI, or a stored file.