API docs/examples for std::slice

 - API doc/example for next() in Permutations
 - API doc/example for permutations() in ImmutableCloneableVector
 - Moved examples for permutations and next into trait definition as
   comments on pull request #16244.
 - Fix erroneus inclusion of src/llvm in older commit.
Update docs to use HTTPS for static.rust-lang.org addresses

cc #16123
Download snapshots using HTTPS

cc #16123
Guide: vectors
auto merge of #16429 : steveklabnik/rust/guide_vectors, r=cmr

Since https://github.com/rust-lang/rust/pull/16380 didn't get pulled in yet, I added it in here too.

This covers the very, very, very basics of vectors. I wanted to have a section that mentioned them, but I'm
unsure what else I should cover. So I just did the absolute simplest things. Feedback very welcome.
auto merge of #16427 : brson/rust/https, r=thestinger
auto merge of #16417 : jasonthompson/rust/docs/slice3, r=alexcrichton

- Moved examples for permutations and next into trait definition as
   comments on pull request #16244.
- Fixed (hopefully) issue with erronious commit of changes to src/llvm.
libnative: process spawning should not close inherited file descriptors

* The caller should be responsible for cleaning up file descriptors
* If a caller safely creates a file descriptor (via
  native::io::file::open) the returned structure (FileDesc) will try to
  clean up the file, failing in the process and writing error messages
  to the screen.
* This should not happen as the caller has no public interface for
  telling the FileDesc structure to NOT free the underlying fd.
* Alternatively, if another file is opened under the same fd held by
  the FileDesc structure returned by native::io::file::open, it will
  close the wrong file upon destruction.