Changes to the ticket system

[Simon Marlow]

I've made a few changes to the ticket system today, in preparation for a 
push on cleaning up the ticket database (announcement to be made 
shortly).  The changes are:

1. There's a new field "Type of Failure", with values

None/Unknown
Build failure
Install failure
GHC doesn't work at all
Compile-time crash
Runtime crash
GHCi crash
Incorrect result at runtime
Compile-time performance bug
Runtime performance bug

Hopefully the values are self-explanatory.  Setting this correctly will 
help us to focus on important tickets, and may help us automatically 
prioritize tickets in the future.

2. The "Severity" field has gone away.  This field was intended to be 
used as a way for ticket submitters to communicate to us the importance 
of the bug to them, but in practice it wasn't clear how to use it, it 
hasn't been used consistently, and as a result we generally ignore it. 
The new "Type of Failure" is a much less ambiguous way to communicate 
some of the same information.

3. The "Difficulty" field has more sensible values representing a range 
of times, rather than just "1 hour" or "1 week":

Unknown
Easy (less than 1 hour)
Moderate (less than a day)
Difficult (2-5 days)
Project (more than a week)
Rocket Science


Cheers,
	Simon