initial impl of default type args with typechecker phase-related bug
attempt to fix lifecycle bug with default type args, still very fragile 
handle type argument defaults in ProducedType
union/intersection canonicalization still broken
Fix invalid annotation example.

Now resembles the example found on the ceylon-lang.org introduction
page.
fix #494 transitive visibility checking for union type aliases
make is/exists/nonempty operators postfix, according to #496, #257
updated grammar for "given var = expr then ...", "exists expr then ...",
etc
support for => in named arg lists to fix #466
support for => in named arg lists to fix #466
fix #497
make sequenced args in named args lists arguments to params of type
Iterable fix #465
another test
extra test
Use angle brackets for grouping in type expressions #460
Revert "Use angle brackets for grouping in type expressions #460"

This reverts commit bdeebf3f107775e3a85045e0a54842488b1b551d.
syntactic changes for #488, #460
fix #489 by removing -> abbreviation of Entry types
remove obsolete ? binary operator
update tests for changes to Ranged
fix #467
reintroduce X->Y entry type abbreviation, but with correct precedence
this time #489
change algorithm for abbreviating types to use grouping < >
abbreviate Empty to []
abbreviate Entry<X,Y> to X->Y