Make the `const_err` lint `deny`-by-default

At best these things are runtime panics (debug mode) or overflows (release mode). More likely they are public constants that are unused in the crate declaring them.

This is not a breaking change, as dependencies won't break and root crates can `#![warn(const_err)]`, though I don't know why anyone would do that.

add auto-impl for primitive type

Part of #50431.

I have no clue how to test this though with the rustdoc test suite...

r? @QuietMisdreavus

Implement edition hygiene for keywords

Determine "keywordness" of an identifier in its hygienic context.
cc https://github.com/rust-lang/rust/pull/49611

I've resurrected `proc` as an Edition-2015-only keyword for testing purposes, but it should probably be buried again. EDIT: `proc` is removed again.

Stabilise inclusive_range_methods

r? @SimonSapin

Closes #49022.

Update RLS and Rustfmt

Fixes RLS build (The Rustfmt update is insignificant)

r? @alexcrichton

Rollup of 10 pull requests

Successful merges:

 - #50387 (Remove leftover tab in libtest outputs)
 - #50553 (Add Option::xor method)
 - #50610 (Improve format string errors)
 - #50649 (Tweak `nearest_common_ancestor()`.)
 - #50790 (Fix grammar documentation wrt Unicode identifiers)
 - #50791 (Fix null exclusions in grammar docs)
 - #50806 (Add `bless` x.py subcommand for easy ui test replacement)
 - #50818 (Speed up `opt_normalize_projection_type`)
 - #50837 (Revert #49767)
 - #50839 (Make sure people know the book is free oline)

Failed merges:

Streamline `StringReader::bump`

These patches make `bump` smaller and nicer. They speed up most runs for coercions and tuple-stress by 1--3%.

stop considering location when computing outlives relationships

This doesn't (yet?) use SEME regions, but it does ignore the location for outlives constraints. This makes (I believe) NLL significantly faster -- but we should do some benchmarks. It regresses the "get-default" family of use cases for NLL, which is a shame, but keeps the other benefits, and thus represents a decent step forward.

r? @pnkfelix

Make sure people know the book is free oline

I've used the tutorial a number of times to relearn rust basics.  When i saw this for a moment I was sad thinking it had been taken offline.

Revert #49767

There was [some confusion](https://github.com/rust-lang/rust/pull/49767#issuecomment-389250815) and I accidentally merged a PR that wasn't ready.

Speed up `opt_normalize_projection_type`

`opt_normalize_projection_type` is hot in the serde and futures benchmarks in rustc-perf. These two patches speed up the execution of most runs for them by 2--4%.

Add `bless` x.py subcommand for easy ui test replacement

fixes #49815

r? @nikomatsakis

Fix null exclusions in grammar docs

The grammar documentation incorrectly says that comments, character literals,
and string literals may not include null.

Fix grammar documentation wrt Unicode identifiers

The grammar defines identifiers in terms of XID_start and XID_continue,
but this is referring to the unstable non_ascii_idents feature.
The documentation implies that non_ascii_idents is forthcoming, but this
is left over from pre-1.0 documentation; in reality, non_ascii_idents
has been without even an RFC for several years now, and will not be
stabilized anytime soon. Furthermore, according to the tracking issue at
https://github.com/rust-lang/rust/issues/28979 , it's highly
questionable whether or not this feature will use XID_start or
XID_continue even when or if non_ascii_idents is stabilized.
This commit fixes this by respecifying identifiers as the usual
[a-zA-Z_][a-zA-Z0-9_]*

Tweak `nearest_common_ancestor()`.

- Remove the "no nearest common ancestor found" case, because it's never
  hit in practise. (This means `closure_is_enclosed_by` can also be
  removed.)

- Add a comment about why `SmallVec` is used for the "seen" structures.

- Use `&Scope` instead of `Scope` to avoid some `map()` calls.

- Use `any(p)` instead of `position(p).is_some()`.

r? @nikomatsakis

Improve format string errors

Point at format string position inside the formatting string:
```
error: invalid format string: unmatched `}` found
  --> $DIR/format-string-error.rs:21:22
   |
LL |     let _ = format!("}");
   |                      ^ unmatched `}` in format string
```

Explain that argument names can't start with an underscore:
```
error: invalid format string: invalid argument name `_foo`
  --> $DIR/format-string-error.rs:15:23
   |
LL |     let _ = format!("{_foo}", _foo = 6usize);
   |                       ^^^^ invalid argument name in format string
   |
   = note: argument names cannot start with an underscore
```

Fix #23476.

The more accurate spans will only be seen when using `format!` directly, when using `println!` the diagnostics machinery makes the span be the entire statement.

Add Option::xor method

Implements the method requested in #50512.

Remove leftover tab in libtest outputs

This removes some tabs that were present in the output of libtest.

Related #19299
Closes #50362

There was [some confusion](https://github.com/rust-lang/rust/pull/49767#issuecomment-389250815) and I accidentally merged a PR that wasn't ready.

Switch to bootstrapping from 1.27

It's possible the Float trait could be removed from core, but I couldn't tell whether it was intended to be removed or not. @SimonSapin may be able to comment more here; we can presumably also do that in a follow up PR as this one is already quite large.