Honor hidden doc attribute of derivable trait methods

Closes #13698
Remove special rooting code from trans

[breaking-change]

cc #11586
rustc: remove usage of root_map from astencode
rustc: Remove root_map usage from _match
rustc: Remove root_map usage from lifetime
rustc: Remove root_map entirely
Update tests and move other tests around
rustc: Remove obsolete error_out_of_root_scope variant
Treat @T like ~T in borrowck

`@` pointers used to have special rooting and regions management. With `@`
moving to standalone library, we don't need to keep that special
treatment around. This patch modifies the way `@` pointers are treated by
treating them as if they were `~` pointers

Region checker and borrow checker were modified in this patch.

Closes #11586

[breaking-change]
rustc: Remove moved_variables_set
syntax: fix de-@rooting fallout
rustc: fix de-@rooting fallout
rustc: Fix passing errors from LLVM to rustc

Many of the instances of setting a global error variable ended up leaving a
dangling pointer into free'd memory. This changes the method of error
transmission to strdup any error and "relinquish ownership" to rustc when it
gets an error. The corresponding Rust code will then free the error as
necessary.

Closes #12865
rustc: Give a friendlier error when writing deps

When an error is encountered when writing dependencies, this presents a nicer
error rather than an ICE.

Closes #13517
rustc: Don't die when a crate id can't be inferred

The filestem of the desired output isn't necessarily a valid crate id, and
calling unwrap() will trigger an ICE in rustc. This tries a little harder to
infer a "valid crate id" from a crate, with an eventual fallback to a generic
crate id if alll else fails.

Closes #11107
fix std::f32 and std::f64 constants

Some of the constant values in std::f32 were incorrectly copied from
std::f64.  More broadly, both modules defined their constants redundantly
in two places, which is what led to the bug.  Moreover, the specs for
some of the constants were incorrent, even when the values were correct.

Closes #13297.  Closes #11537.
Calibrate span for method call error messages

Specifically, the method parameter cardinality mismatch or missing
method error message span now gets method itself exactly. It was the
whole expression.

Closes #9390
Closes #13684
Closes #13709
Prevent unsized types being stored in variables

Closes #13376.
Move task::task() to TaskBuilder::new()

The constructor for `TaskBuilder` is being changed to an associated
function called `new` for consistency with the rest of the standard
library.

Closes #13666

[breaking-change]
auto merge of #13675 : sfackler/rust/taskbuilder-new, r=alexcrichton

The constructor for `TaskBuilder` is being changed to an associated
function called `new` for consistency with the rest of the standard
library.

Closes #13666

[breaking-change]