Add DefaultedType to compileTypeIsh

(I found this because Error was removed and it didn’t compile – see also
the next commit – but why change it to AssertionError if the error is
long outdated anyway, and the feature can actually be implemented? :) )
Use AssertionError instead of Error

(Error was removed)
Add ArgumentList, SpreadArgument

ArgumentList is RedHat’s SequencedArgument – RedHat’s ArgumentList is
the superclass of NamedArgumentList, PositionalArgumentList (which I
will probably suffix -Arguments instead, after the spec).
Add tests for ArgumentList, SpreadArgument backend conversion
Use tuple type syntax sugar

I’m about to add a Tuple node type, so we need to avoid this conflict :)

(Another possibility would be to import Tuple from ceylon.language,
which seems to override the package-local Tuple.)
Add Iterable, Tuple

For some reason, RedHat’s Iterable is called SequenceEnumeration.
Add tests for Iterable, Tuple backend conversion