vm support for exceptions

[mbeckem]

Exceptions are already supported by the compiler and must be implemented in the vm.

• Correct unwinding behaviour, i.e. call exception handlers and transport exceptions objects
• Unhandled exceptions terminate the current coroutine, the exception object is used to set the coroutine's result (to error)
• Provide a magic std.pcall(fn) (name TBD) function that can catch exceptions thrown by fn