Fix ICEs that involved quasi-quotation

* fixed get_tt for doc comments
* properly handle MatchNt in `quote`

Fixes #18763
Fixes #18775
Fix broken documentation link
Add regression test for #18566
Add optional messages to the unreachable macro.

Closes #18842.
Set exceeding bitshifts lint to deny

Discussed in #18587
test: Rename files, fail -> panic.
Fix remaining documentation to reflect fail!() -> panic!()

Throughout the docs, "failure" was replaced with "panics" if it means a
task panic. Otherwise, it remained as is, or changed to "errors" to
clearly differentiate it from a task panic.
libsyntax: Forbid type parameters in field expressions.

This breaks code like:

    struct Foo {
        x: int,
    }

    let f: Foo = ...;
    ... f.x::<int> ...

Change this code to not contain an unused type parameter. For example:

    struct Foo {
        x: int,
    }

    let f: Foo = ...;
    ... f.x ...

Closes #18680.

[breaking-change]
Fix documentation bug

The first paragraph must be separated from the next paragraph.
Otherwise, rustdoc will consider the content of the latter as part of
the title.
rustc: do not inherit #[stable]

This patch tweaks the stability inheritance infrastructure so that
`#{stable]` attributes are not inherited. Doing so solves two problems:

1. It allows us to mark module *names* as stable without accidentally
marking the items they contain as stable.

2. It means that a `#[stable]` attribution must always appear directly
on the item it applies to, which makes it easier for reviewers to catch
changes to stable APIs.

Fixes #17484
Update tests for new stability inheritance rules
std: Rename AsRef{Reader,Writer} to ByRef{Reader,Writer}
Fix inconsistent use of substs in trans_unboxing_shim

Substs were not applied when calling `untuple_arguments_if_necessary`.
Just apply them once at the start of the function, rebinding `fty`.
Also change the function to take them by reference since we don't
need to consume them at all.  Closes #18883
Add regression test for #18883
Results aren't panics

A typo about Results being panics crawled in. Fixing it.
Take parameters by-value in Signed trait
Deprecate Signed method wrappers
Move abs_sub to FloatMath

This removes the need for libcore to depend on libm. `abs_sub` is not as useful for integers.
Create UnsignedInt trait and deprecate free functions
Move saturating operator methods into Int