Fix po4a conf file

This line was declared twice, which causes the build of i10n docs to
fail.
Fix inappropriate ## headings

Fixes #15499.
Fix up some {ignore} and {notrust}s

These should be properly annotated instead.

Fixes #16219.
Update intro.md
gdb < RUST_BACKTRACE

Fixes #19717
reference: type definition -> type alias

Fixes #9266
Remove mentions of sundown.
Add doctests for `iter` and `into_iter` for BTreeSet.
rustdoc: Properly inline const items

Build `clean::ConstantItem` values in the `inline` module and
pretty-print the AST for inlined const items.

Doc strings are still missing from inlined constants (see #19773).

Partially address #18156, #19722, #19185

Fix #15821
gedit language spec: add new-style Unicode escapes
Add Ord impl to raw pointers
Remove feature gate directives for `if_let`, `while_let`, and `tuple_indexing`.
Update vim syntax highlighting for ExactSizeIterator
libc::c_char is not necessarily i8
metadata: Encode attributes for const items

Fix #19773

Together with #19774 (which this is rebased on):

Fix #18156, fix #19722, fix #19185
Add a doctest for BTreeSet::new.
vim: Support the new \u{1234} unicode escapes
Make BTree's Handle system more generic and more powerful, removing some runtine checks in favor of newly gained static safety
Expansion should explicitly include enum

In preparation for removing the std::cmp::Ordering reexport, this needs
to be done to prevent errors like:

```
note: in expansion of #[deriving]
note: expansion site
error: unresolved name `std::cmp::Equal`
\#[deriving(Clone, PartialEq, PartialOrd, Eq, Ord, Show)]
                                              ^~~
```
Fix a typo in ownership guide, aquire to acquire.