doc: make the sentence make more sense
Guide: generics
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
Use byte literals in libcollections tests
Use byte literals in libserialize
Use byte literals in libdebug
Use a byte literal in libregex
Use byte literals in libterm
Allow generic foreign functions.

Generic extern functions written in Rust have their names mangled, as well as their internal clownshoe __rust_abi functions. This allows e.g. specific monomorphizations of these functions to be used as callbacks.

Closes #12502.
Add new tests for extern and foreign fns and name mangling.
Use byte literal in libnum
Use byte literals in libsyntax
Use byte literals in libstd
Use byte literals in libcore
core: Refactor iterators

Simplifying the code of methods: nth, fold, rposition
and iterators: Filter, FilterMap, SkipWhile
Adding basic benchmarks
AST refactoring: merge PatWild and PatWildMulti into one variant with a flag.
Remove cast to char in libserialize::hex
Use byte literals in libgetopts
make rustdoc more responsive

* move some sidebar contents to a title bar when small
* inline description toggle when small
* make out-of-band and in-band content share space, rather than float and clash
* compress wording of out-of-band content to avoid line-wrap as much as possible
auto merge of #16225 : pczarn/rust/iter-refactoring, r=kballard

Simplifying the code of methods: `nth`, `fold`, `rposition`, and iterators: `Filter`, `FilterMap`, `SkipWhile`.

```
before
test iter::bench_multiple_take      ... bench:        15 ns/iter (+/- 0)
test iter::bench_rposition          ... bench:       349 ns/iter (+/- 94)
test iter::bench_skip_while         ... bench:       158 ns/iter (+/- 6)

after
test iter::bench_multiple_take      ... bench:        15 ns/iter (+/- 0)
test iter::bench_rposition          ... bench:       314 ns/iter (+/- 2)
test iter::bench_skip_while         ... bench:       107 ns/iter (+/- 0)
```
@koalazen has the code for `Skip`.

Once #16011 is fixed, `min_max` could use a for loop.