Previously, the error messages could be shown out of order.
With this patch, the errors are sorted before being shown to the user.
This makes it easier to read the error messages, and matches what the
`go` toolchain does.
This is a refactor, which should (in theory) not change the behavior of
the compiler. But since this is a pretty large change, there is a chance
there will be some regressions. For that reason, the previous commits
added a bunch of tests to make sure most error messages will not be
changed due to this refactor.
Ayke van Laethem