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
Fix #19734 (ICE)
Add test for issue #19734
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