Add next_permutation and prev_permutation onto MutableTotalOrdVector<T>.

Unlike ImmutableClonableVector::permutations() which returns an iterator,
cloning the entire array each iteration, these methods mutate the vector in-place.
For that reason, these methods are much faster; between 35-55 times faster,
depending on the benchmark. They also generate permutations in lexicographical order.
test: Enable extern-fn-reachable test

It didn't work because it tried to call itself but symbols are not
exported as default in executables.

Note that `fun5` is not internal anymore since it is in library.
test: Remove/update some old ignored tests
add min_pos_value constant for floats

Follow-up on issue #13297 and PR #13710.  Instead of following the (confusing) C/C++ approach
of using `MIN_VALUE` for the smallest *positive* number, we introduce `MIN_POS_VALUE` (and
in the Float trait, `min_pos_value`) to represent this number.

This patch also removes a few remaining redundantly-defined constants that were missed last
time around.
rustdoc: fixes
doc: slight design refresh
doc,rustdoc: store webfonts locally

- Avoids cross-domain requests restrictions
- Better availability of content
- No HTML queries needed for an offline build
rustdoc: bring it inline
doc: Update modules for containers
clarify docs for std:io::fs::Path::{is_dir,is_file,exists}; add lstat

Clarifies the interaction of `is_dir`, `is_file` and `exists` with
symbolic links.  Adds a convenience `lstat` function alongside of
`stat`.  Removes references to conditions.

Closes issue #12583.
Update FFI signature table to use pipe format

Rustdoc doesn't seem like it's converting the old format to a table as we can see here:
http://static.rust-lang.org/doc/master/complement-cheatsheet.html#ffi-(foreign-function-interface)
This new format should fix that and it's also rendered by Github's markdown preview.
Tests for dynamic regexes will now run during 'check-stage2'.

Before, tests for dynamic regexes ran during stage1 and tests for
native regexes ran during stage2. But the buildbots don't test stage1,
so now both dynamic and native tests are run during stage2.

Closes #13740.
rustc: Restrict the scope of a borrow on def_map

This addresses the ICE from #13763, but it does not allow the test to compile,
due to #13768. An alternate test was checked in in the meantime.

Closes #13763
syntax: ViewItemUse no longer contains multiple view paths.

it reflected the obsolete syntax `use a, b, c;` and did not make
past the parser (though it was a non-fatal error so we can continue).
this legacy affected many portions of rustc and rustdoc as well,
so this commit cleans them up altogether.
rustdoc: Moved `SCHEMA_VERSION` to `clean`.

it should be changed when the module gets updated, so it helps to
have the `SCHEMA_VERSION` with the definitions themselves.
Fixing permutation of small lists, such that [], [x] -> [[]], [[x]], and updating size_hints.
Fixes #13734 and #13759.
Fix link to hashmap.rs and json.rs in sample code FAQ
Fixed typo in std::iter
Rewrote documentation for parse_bytes and to_str_bytes in {int, uint}_macros.rs
Added missing values in std::io::standard_error()