Automatically export methods on core numeric types

[bstrie]

Each numeric type now contains an extensions module that is automatically exported. At the moment each extensions module contains only the impl for the num::num iface. Other impls soon to follow (hopefully), partly contingent on #2716 being fixed.

[brson]

I don't like this extra 'extensions' module, so I made an additional commit to remove it again, while keeping num exported:

https://github.com/brson/rust/tree/tmp

Testing this on 'try' now. Looking into #2716.

[brson]

pushed to incoming

[bstrie]

@brson The only reason that I opted to stick all the impls in an "extensions" mod was in an effort to not pollute the top-level index for the libcore docs (i.e. there'd just be "sometype::extensions" for each type, rather than "sometype::num, sometype::ord, sometype::eq")... but I'm not certain that I even accomplished that.

[brson]

Oh, I remember you telling me that now. I thought it was to work around resolve errors.

I don't think that approach will actually work, since core.rs exports extensions::*, effectively pulling all those things into the the top level anyway.