Improve messages during verifyproblem runs

[gkreitz]

I like how judging flashes the test case it's currently working on on the screen. This functionality is now a part of Diagnostics, and also used for rendering statements and running input validators.

I also added back msg to Diagnostics, replacing print in checks. This allows the flashing message functionality to work correctly even if someone does a print while a message is shown. It also allows API users to catch print:s made by the problem verification.

For most parts we check, we now print a bit more information (ends up being a bit of bespoke code to format the message, but I think it's worth it). E.g.,

$ verifyproblem.sh testthelist/ -d secret/01-correct -s accepted
Loading problem testthelist
Checking config
Checking problem statements in 2 languages
Checking 1 input validator                      
Checking output validator
Checking 2 test cases in 3 test data groups (ignoring 16 cases due to filters)
Checking include files for 1 language (1 overriding entrypoint)
Checking 3 submissions (ignoring 1 submission due to filters)