Implement type 'any' and alt-type

[froystig]

Similar to alt but branches on the actual type of its any-typed head expression rather than the value. Implement in full --- doesn't even parse at the moment. See manual. Should also include the 'any' type and how to inject/extract values from it.

[graydon]

Shifted to rustc

[brson]

To really make this useful now it would need to have variations for the different kinds, any~, any@, any&, or such.

[brson]

Even though we haven't had a pressing need for this I think it could be useful in error handling. For example an error(any) type is more composable than error(T). And given that fail essentially takes type any, it would provide a nice way to perform a 'try' operation in another task and return an error(any~) of the fail value to the original task.

[catamorphism]

#1498 -- as well as traits/impls -- supersede this, we agreed. Closing.