This comit implements a new flag, --extern, which is used to specify where a
crate is located. The purpose of this flag is to bypass the normal crate
loading/matching of the compiler to point it directly at the right file.

This flag takes the form `--extern foo=bar` where `foo` is the name of a crate
and `bar` is the location at which to find the crate. Multiple `--extern`
directives are allowed with the same crate name to specify the rlib/dylib pair
for a crate. It is invalid to specify more than one rlib or more than one dylib,
and it's required that the crates are valid rust crates.

I have also added some extensive documentation to metadata::loader about how
crate loading should work.

RFC: 0035-remove-crate-id

The compiler will no longer insert a hash or version into a filename by default.
Instead, all output is simply based off the crate name being compiled. For
example, a crate name of `foo` would produce the following outputs:

* bin => foo
* rlib => libfoo.rlib
* dylib => libfoo.{so,dylib} or foo.dll
* staticlib => libfoo.a

The old behavior has been moved behind a new codegen flag,
`-C extra-filename=<hash>`. For example, with the "extra filename" of `bar` and
a crate name of `foo`, the following outputs would be generated:

* bin => foo (same old behavior)
* rlib => libfoobar.rlib
* dylib => libfoobar.{so,dylib} or foobar.dll
* staticlib => libfoobar.a

The makefiles have been altered to pass a hash by default to invocations of
`rustc` so all installed rust libraries will have a hash in their filename. This
is done because the standard libraries are intended to be installed into
privileged directories such as /usr/local. Additionally, it involves very few
build system changes!

RFC: 0035-remove-crate-id
[breaking-change]

This commit modifies crate loading to purely work off a `crate_name` and nothing
else. This commit also changes the patterns recognized from `lib<foo>-*` to
`lib<foo>*` to accomodate the future renamings of output files.

RFC: 0035-remove-crate-id

This metadata is used to drive the hash of all symbols in a crate. The flag can
be specified a multiple number of times

RFC: 0035-remove-crate-id

This commit removes all support in the compiler for the #[crate_id] attribute
and all of its derivative infrastructure. A list of the functionality removed is:

* The #[crate_id] attribute no longer exists
* There is no longer the concept of a version of a crate
* Version numbers are no longer appended to symbol names
* The --crate-id command line option has been removed

To migrate forward, rename #[crate_id] to #[crate_name] and only the name of the
crate itself should be mentioned. The version/path of the old crate id should be
removed.

For a transitionary state, the #[crate_id] attribute is still accepted if
the #[crate_name] is not present, but it is warned about if it is the only
identifier present.

RFC: 0035-remove-crate-id
[breaking-change]

Fix small bug introduced in e38cb972: PatIdent span was incorrect
because self.last_span was being used before the ident token was
parsed.

Inadvertently changed the order in which destructors ran in certain cases with #15076.

Fixes #15438.

In my informal measurements, this brings the peak memory usage when
building librustc from 1662M down to 1502M.  Since 1662 - 1502 = 160,
this may not recover the entirety of the observed memory regression
(250M) from PR #14604.  (However, according to my local measurements,
the regression when building librustc was more like 209M, so perhaps
this will still recover the lions share of the lost memory.)

The types `Bitv` and `BitvSet` are badly out of date. This PR:
- cleans up the code (primarily, simplifies `Bitv` and implements `BitvSet` in terms of `Bitv`)
- implements several new traits for `Bitv`
- adds new functionality to `Bitv` and `BitvSet`
- replaces internal iterators with external ones
- updates documentation
- minor bug fixes

This is a significantly souped-up version of PR #15139 and is the result of the discussion there.

In my informal measurements, this brings the peak memory usage when
building librustc from 1662M down to 1502M.  Since 1662 - 1502 = 160,
this may not recover the entirety of the observed memory regression
(250M) from PR #14604.  (However, according to my local measurements,
the regression when building librustc was more like 209M, so perhaps
this will still recover the lions share of the lost memory.)

This basically meant changing the interface so that no borrowed `&Vec`
is exposed, by hiding `fn get_vec` and `fn get_mut_vec` and revising
`fn all_vecs`.

Instead, clients should use one of the other methods; `get_slice`,
`pop`, `truncate`, `replace`, `push_all`, or `is_empty_in`, which
should work for any case currently used in rustc.

This pull request adds hygiene for 3 kinds of argument bindings:
- arguments to item fns,
- arguments to `ExprFnBlock`s, and
- arguments to `ExprProc`s

It also adds a bunch of unit tests, fixes a few macro uses to be non-capturing, and has a few cleanup items.

local `make check` succeeds.

This speeds up json serialization by removing most of the allocations.

This significantly speeds up encoding json strings.

Fix small bug introduced in e38cb972: PatIdent span was incorrect
because self.last_span was being used before the ident token was
parsed.

also, introduce fn_decl_arg_bindings and expand_and_rename abstractions

This was parsed by the parser but completely ignored; not even stored in
the AST!

This breaks code that looks like:

    static X: &'static [u8] = &'static [1, 2, 3];

Change this code to the shorter:

    static X: &'static [u8] = &[1, 2, 3];

Closes #15312.

[breaking-change]

r? @nick29581