Rename assert_eq arguments to left and right.

The names expected and actual are not used anymore in the output. It also
removes the confusion that the argument order is the opposite of junit.
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
Add comments with type annotations.

This will hopefully help people with their first steps in Rust.

Fixes #16143.
Remove mentions of sundown.
Use rust-installer for installation

This is just a refactoring of the current installer so that Rust and Cargo
use the same codebase.

cc #16456
serialize: Avoid stray nul characters when auto-serializing char.

Fixes #19719.
Add coercions from *mut to *const and from &mut to *const.
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