As part of making tuples first-class citizens, expliciting the arity upon
function application was needed (so that a function of two args can
transparently -- in the surface language -- be applied to either two arguments
or a pair).
It was decided to actually explicit the whole type of arguments because the cost
is the same, and this is consistent with lambda definitions.
A related change done here is the replacement of the `EOp` node for operators by
an "operator application" `EAppOp` node, enforcing a pervasive invariant that
operators are always directly applied. This makes matches terser, and highlights
the fact that the treatment of operator application is almost always different
from function application in practice.
Some typing errors are changed a little, because they get triggered during the
typing of the disambiguation pass, which does not specify the expected return
type (it's an expected invariant that it should not be needed for
disambiguation).
It would be possible to still specify these types during disambiguation just to
get the same errors, but since the newer ones don't appear to be clearly worse
at the moment, it has not been done.
