Avoid building as root with `sudo make install`

When running `sudo make install`, we only want to run the actual install
as root, the building of the documentation and the distribution folder
should happen as the non-root user.

Related to #13728.
Add help diagnostic messages

This adds ‘help’ diagnostic messages to rustc. This is used for anything that
provides help to the user, particularly the `--explain` messages that were
previously integrated into the relevant error message.
Changed `extern crate foo as bar;` error message

Closes #17709
Track kind of closure in upvar categorization

Keep track of the kind of closure responsible for an upvar
Categorize upvars in `Fn` unboxed closures as freely aliasable

This causes borrowck to correctly reject mutation or mutable borrows
of upvars in `Fn` unboxed closures since the closure environment is
aliasable.

This also tracks the responsible closure in the aliasability
information returned and uses it to give a helpful diagnostic.

Closes issue #17780
Fix unit test that was illegally mutating an upvar
Add regression test for issue #17780
Fix target-feature codegen option syntax in rustc man page.

The man page stated that the list of features was space-separated when
it's actually comma-separated.
adding iterator benches to tree-based maps for comparison purposes
flesh out BTree docs
syntax: Parse outer attributes in quote_method!

Fixes #17782.
doc fixups
Properly handle cfgs in rustdoc

Rustdoc would previously improperly handle key="value" style cfgs, which
are notably used for Cargo features.
Clarify that assigning/comparing different tuple types to one another won't compile
Re-exports core::str::Utf16CodeUnits in std::str

Closes #17751
Fix the most egregious instances of "local ambiguity: multiple parsing options..." error in macros, which often occurs when trying to match parts of Rust syntax.
For example, this matcher: `fn $name:ident( $($param:ident : $pty:ty),* )` would fail when parsing `fn foo()`, because macro parser wouldn't realize that an ident cannot start with `)`.

This resolves #5902, and at least partially mitigates #9364 and #3232.
Guide: Fix inconsistency in 'Marcos' section

The old version switched in between examples from the value `5i` to `"Hello"`
and back. Additionally, the code generated by `rustc print.rs
--pretty=expanded` is not as verbose anymore.
Add rustup.sh to the repo

Just to have it somewhere to point to. Updating it will not
automatically update the one on static.rust-lang.org.
Rename RELEASES.txt to RELEASES.md. It's markdown.
Update RELEASES.md for 0.12.0