ast.ml - modified arm types for easier polymorphism
- fixed a bug in fmt_type_arm
dead.ml - modified arm types for easier polymorphism
common.ml - added 'either'
- added some useful auxiliary functions
item.ml - modified arm code to be more polymorphic and handle both alt-tag and alt-type, also fixed the problematic case in bad-alt.rs
Makefile - added XFAIL for new alt-type test
bad-alt.rs - added test for invalid alt syntax
alt-type-simple.rs - added simple test for alt type