Relax lifetime bounds on Reader/Writer impls for trait boxes

Cargo needs this to be able to instantiate `TerminfoTerminal<Box<Writer+'a>>`
for 'a other than 'static.
Fix whitespace and missing parentheses.
std: branchless bucket distance for hashmap
std: RawTable exposes a safe interface for HashMap

Introduced a new growth algorithm.
std: Split hashmap.rs into modules
Auto-derive Clone for syntax::diagnostic::Level.
std: Refine and document HashMap's code

* branchless `bucket.next()`
* robin_hood is a free function
* fixed the resize policy that was off by one
* documented the growth algorithm
* updated documentation after interface changes
* removed old fixmes
std: Fix overflow of HashMap's capacity
Work around inability to link lifetime of ref bindings (#16994)
auto merge of #16990 : DiamondLovesYou/rust/level-derive-clone, r=alexcrichton
auto merge of #16843 : bkoropoff/rust/reader-writer-box, r=alexcrichton

Cargo needs this to be able to instantiate `TerminfoTerminal<Box<Writer+'a>>` for 'a other than 'static.
auto merge of #16775 : jbcrail/rust/fix-rational-docs, r=pcwalton

Minor corrections to Rational documentation.
make CrateContext fields private
split CrateContext into shared and local pieces

Break up `CrateContext` into `SharedCrateContext` and `LocalCrateContext`.  The
local piece corresponds to a single compilation unit, and contains all
LLVM-related components.  (LLVM data structures are tied to a specific
`LLVMContext`, and we will need separate `LLVMContext`s to safely run
multithreaded optimization.)  The shared piece contains data structures that
need to be shared across all compilation units, such as the `ty::ctxt` and some
tables related to crate metadata.
move back::link::write into a separate file
run optimization and codegen on worker threads

Refactor the code in `llvm::back` that invokes LLVM optimization and codegen
passes so that it can be called from worker threads.  (Previously, it used
`&Session` extensively, and `Session` is not `Share`.)  The new code can handle
multiple compilation units, by compiling each unit to `crate.0.o`, `crate.1.o`,
etc., and linking together all the `crate.N.o` files into a single `crate.o`
using `ld -r`.  The later linking steps can then be run unchanged.

The new code preserves the behavior of `--emit`/`-o` when building a single
compilation unit.  With multiple compilation units, the `--emit=asm/ir/bc`
options produce multiple files, so combinations like `--emit=ir -o foo.ll` will
not actually produce `foo.ll` (they instead produce several `foo.N.ll` files).

The new code supports `-Z lto` only when using a single compilation unit.
Compiling with multiple compilation units and `-Z lto` will produce an error.
(I can't think of any good reason to do such a thing.)  Linking with `-Z lto`
against a library that was built as multiple compilation units will also fail,
because the rlib does not contain a `crate.bytecode.deflate` file.  This could
be supported in the future by linking together the `crate.N.bc` files produced
when compiling the library into a single `crate.bc`, or by making the LTO code
support multiple `crate.N.bytecode.deflate` files.
reuse original symbols for inlined items

When inlining an item from another crate, use the original symbol from that
crate's metadata instead of generating a new symbol using the `ast::NodeId` of
the inlined copy.  This requires exporting symbols in the crate metadata in a
few additional cases.  Having predictable symbols for inlined items will be
useful later to avoid generating duplicate object code for inlined items.
translate into multiple llvm contexts

Rotate between compilation units while translating.  The "worker threads"
commit added support for multiple compilation units, but only translated into
one, leaving the rest empty.  With this commit, `trans` rotates between various
compilation units while translating, using a simple stragtegy: upon entering a
module, switch to translating into whichever compilation unit currently
contains the fewest LLVM instructions.

Most of the actual changes here involve getting symbol linkage right, so that
items translated into different compilation units will link together properly
at the end.
avoid duplicate translation of monomorphizations, drop glue, and visit glue

Use a shared lookup table of previously-translated monomorphizations/glue
functions to avoid translating those functions in every compilation unit where
they're used.  Instead, the function will be translated in whichever
compilation unit uses it first, and the remaining compilation units will link
against that original definition.
make symbols internal when possible

Add a post-processing pass to `trans` that converts symbols from external to
internal when possible.  Translation with multiple compilation units initially
makes most symbols external, since it is not clear when translating a
definition whether that symbol will need to be accessed from another
compilation unit.  This final pass internalizes symbols that are not reachable
from other crates and not referenced from other compilation units, so that LLVM
can perform more aggressive optimizations on those symbols.