native, rustuv: Fix spawning with empty args

There was a bug in both libnative and libuv which prevented child processes from
being spawned correctly on windows when one of the arguments was an empty
string. The libuv bug has since been fixed upstream, and the libnative bug was
fixed as part of this commit.

When updating libuv, this also includes a fix for #15149.

Closes #15149
Closes #16272
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.
auto merge of #16284 : alexcrichton/rust/issue-16272, r=aturon

There was a bug in both libnative and libuv which prevented child processes from
being spawned correctly on windows when one of the arguments was an empty
string. The libuv bug has since been fixed upstream, and the libnative bug was
fixed as part of this commit.

When updating libuv, this also includes a fix for #15149.

Closes #15149
Closes #16272
guide-runtime.md: remove redundant verb
guide-testing.md: add auxiliary verb
guide-unsafe.md: fix noun