Move `Duration` type to libcore

[clarfonthey]

Right now there isn't anything that requires keeping this type in libstd, and it may be useful for libraries that work with time in a no_std context. It makes sense to move this type to libcore.

[bstrie]

I thought that all types in libstd were there for a reason, is there a historical reason that this was left in? Should we audit libstd for other types that don't need to be there?

[clarfonthey]

@bstrie everything in libcore is also in libstd, more or less. libcore is just a smaller version of libstd that doesn't rely on platform-specific behaviour at all.

I do think that libstd should be audited for types that can be moved to libcore; this is just one case of that.

[sfackler]

AFAIK it's only in std because the rest of the time module was defined there, and Instant and SystemTime both need std. Seems fine to move Duration though.

[Mark-Simulacrum]

I think a PR for this would be reasonable. Marking as feature-accepted.