This almost removes the StringRef wrapper, since all strings are
Equiv-alent now. Removes a lot of `/* bad */ copy *`'s, and converts
several things to be &'static str (the lint table and the intrinsics
table).

There are many instances of .to_managed(), unfortunately.

Part of #3853

Fix a laundry list of warnings involving unused imports that glutted
up compilation output.  There are more, but there seems to be some
false positives (where 'remedy' appears to break the build), but this
particular set of fixes seems safe.

are currently inferred.  New rules are coming that will require
them to be explicit.  All add some explicit self declarations.

correct handling of moves for struct-record update.

Part of #3678.  Fixes #2828, #3904, #4719.

LinearMap is quite a bit faster, and is fully owned/sendable without
requiring copies. The older std::map also doesn't use explicit self and
relies on mutable fields.

"Dual impls" are impls that are both type implementations and trait
implementations. They can lead to ambiguity and so this patch removes them
from the language.

This also enforces coherence rules. Without this patch, records can implement
traits not defined in the current crate. This patch fixes this, and updates
all of rustc to adhere to the new enforcement. Most of this patch is fixing
rustc to obey the coherence rules, which involves converting a bunch of records
to structs.

Revert "librustc: Make unqualified identifier searches terminate at the nearest module scope. r=tjc"

This reverts commit a8d37af2.

librustc: Terminate name searches at the nearest module scope for paths that contain at least two components. r=graydon