The required totality of interface methods is now only affected if
there's an explicit modifier on the method. This allows us to set
%default total on the Prelude, which is a good thing to do anyway,
without also requiring that every implementation of the interface in the
prelude has to be total, which would potentially be a pain.
Another good affect is that it speeds up totality checking elsewhere
because totality checking is done lazily, and so with the total flag set
we know in advance that prelude functions are total.
If we're going to use assert_total. we might as well not have the
distinction between empty and non empty results, which only makes the
code harder to follow.
Also there's some kind of issue which seems to mean the token list can't
be erased if we do that. I will investigate if I can make a smaller
example.
Still a couple of things to resolve in coverage and totality checking
before we can switch on %default, so don't expect quite the right
behaviour just yet. More progress though!
Also working on this has caught a few totality errors in the Idris 2
code base that Idris 1 missed... so these are fixed on the way.
This means temporarily removing some '%default total' from the libraries
and tests, since the input for codata checking isn't right yet (it needs
appropriate use of inlining)
