Add Any type, use for task failure and Result

[brson]

This needs further debate.

The most obvious use case for ~Any is for dealing with arbitrarily typed error values. Add it to std, use it for task failure.

Consider changing the Err variant of Result to take ~Any. This would allow monad-style error chaining to work much better since all Results have the same error type.

cc #61, #2408

[brson]

Nominating.

[Kimundi]

For future reference, a possible implementation of ~Any can be seen here: https://gist.github.com/Kimundi/6802198

Right now it uses the Type descriptor pointer as a "Type ID", but for proper cross-crate support it needs a proper intrinsic for that.

Also, for a ~Any to be convenient, there needs to be a way to cast trait objects to other trait objects provided the boxed type implements both. At least upcasting should be supported.

Use case is, eg, for a &Any to be cast to &ToStr for enabling dynamic printing of their value, or in the case of inheritance allow casting a &AnyToStr with trait AnyToStr: Any+ToStr to &Any.

[brson]

We might actually want a more specific dynamic ~Error for this use case.

[catamorphism]

1.0, backcompat

[pnkfelix]

(fixing this is part of our error-handling story, which in turn is related to #9795 )

[eddyb]

I found it odd there is no mention of #9967 in here, is there any reason for that, or was it just overlooked?

[Kimundi]

Just overlooked, someone should close this issue.