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.
Fix several issues with the struct and enum representability-checking logic.
Tweak copy and formatting of Tasks guide
bench: Improve the spectralnorm shootout benchmark

This improves the spectralnorm shootout benchmark through a few vectors after
looking at the leading C implementation:

* The simd-based f64x2 is now used to parallelize a few computations
* RWLock usage has been removed. A custom `parallel` function was added as a
  form of stack-based fork-join parallelism. I found that the contention on the
  locks was high as well as hindering other optimizations.

This does, however, introduce one `unsafe` block into the benchmarks, which
previously had none.

In terms of timings, the before and after numbers are:

```
$ time ./shootout-spectralnorm-before
./shootout-spectralnorm-before  2.07s user 0.71s system 324% cpu 0.857 total
$ time ./shootout-spectralnorm-before 5500
./shootout-spectralnorm-before 5500  11.88s user 1.13s system 459% cpu 2.830 total
$ time ./shootout-spectralnorm-after
./shootout-spectralnorm-after  0.58s user 0.01s system 280% cpu 0.210 tota
$ time ./shootout-spectralnorm-after 5500
./shootout-spectralnorm-after 5500  3.55s user 0.01s system 455% cpu 0.783 total
```
Improve the error message for missing else clauses in if expressions
Only consider impliciy unboxed closure impl if the obligation is
actually for `Fn`, `FnMut`, or `FnOnce`.

Fixes #18019
Change _rhs to _index.

Because var is provided as an index not a right hand side.
Remove unnecessary ::core::ops::

It shows error: failed to resolve. Maybe a missing `extern crate core`
anyway.
Change `from` and `to` to `_from` and `_to`

This to avoid warning: unused variable: `from`,
#[warn(unused_variable)] on by default.
improve shootout-reverse-complement.rs using unsafe code
Fix the description of raw byte string literals
Use the correct LLVM integer sizes

Use the integer sizes LLVM uses, rather than having random projections
laying around. Sizes are u64, Alignments are u32, C_*int is target-dependent
but 64-bit is fine (the int -> C_int conversion is non-precision-losing,
but it can be preceded by `as int` conversions which are, so it is
somewhat ugly. However, being able to suffix a `u` to properly infer
integer types is nice).
Add a compile-time error when oversized types are used

LLVM generates wrong code (which may be an instance of compile-time UB) when
faced with types that take lots of memory - bigger than the address space.
Make using such types a trans error. While trans errors are bad, overbig
types are expected to be very rare.
add some tests
export *IntSlice in libcollections
Renamed AsciiStr::to_lower and AsciiStr::to_upper
Now AsciiStr::to_lowercase and AsciiStr::to_uppercase to match Ascii trait.
[breaking-change]
response for review comments
prevent the creation of integers too big for the target architecture
improve the comments on integer overflow safety