Place `pub` or `priv` on individual items instead.

Fixes most of #4989. I didn't add Persistent{Set,Map} since the only
persistent data structure is fun_treemap and its functionality is
currently too limited to build a trait out of.

To remove the environment pointer, support for function pointers without
an environment argument is needed (i.e. a fixed version of #6661).

Towards #4812. Also includes some minor cleanups.

This only changes the directory names; it does not change the "real"
metadata names.

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

That will, sadly, require one more snapshot.

correct handling of moves for struct-record update.

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

"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.

Because of macros, #[allow(structural_records]] in
extfmt, gc, os, pipes, and run. Will need a snapshot.

This makes the module much more discoverable, and is in line with the
'treemap' naming.

After a snapshot, we can take out the set_buffer_ version (pipes
syntax exts use it).

As per #4421

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

This reverts commit a8d37af2.