Add support for arbitrary flags to MemoryMap.

This also fixes up the documentation a bit, it was subtly incorrect.
Use `mmap` to map in task stacks and guard page

Also implement caching of stacks.
Update task-perf-one-million
Add benchmarks
Fix zero-sized memory mapping
Implement barrier.
auto merge of #11725 : derekchiang/rust/add-barrier, r=alexcrichton
Grammatical fixes.
Simplify and rename macro API

Now that procedural macros can be implemented outside of the compiler,
it's more important to have a reasonable API to work with. Here are the
basic changes:

* Rename SyntaxExpanderTTTrait to MacroExpander, SyntaxExpanderTT to
    BasicMacroExpander, etc. I think "procedural macro" is the right
    term for these now, right? The other option would be SynExtExpander
    or something like that.

* Stop passing the SyntaxContext to extensions. This was only ever used
    by macro_rules, which doesn't even use it anymore. I can't think of
    a context in which an external extension would need it, and removal
    allows the API to be significantly simpler - no more
    SyntaxExpanderTTItemExpanderWithoutContext wrappers to worry about.
Stop allocating so much in arena tests

Android bots are OOMing.
auto merge of #11799 : comex/rust/patch-2, r=brson
auto merge of #11803 : sfackler/rust/simple-mac, r=brson

Now that procedural macros can be implemented outside of the compiler,
it's more important to have a reasonable API to work with. Here are the
basic changes:

* Rename SyntaxExpanderTTTrait to MacroExpander, SyntaxExpanderTT to
    BasicMacroExpander, etc. I think "procedural macro" is the right
    term for these now, right? The other option would be SynExtExpander
    or something like that.

* Stop passing the SyntaxContext to extensions. This was only ever used
    by macro_rules, which doesn't even use it anymore. I can't think of
    a context in which an external extension would need it, and removal
    allows the API to be significantly simpler - no more
    SyntaxExpanderTTItemExpanderWithoutContext wrappers to worry about.
std,extra: Make some types public and other private.

These are either returned from public functions, and really should
appear in the documentation, but don't since they're private, or are
implementation details that are currently public.
auto merge of #11805 : brson/rust/smallerarenas, r=brson

Android bots are OOMing.
auto merge of #11808 : huonw/rust/std-visible-types, r=brson

These are either returned from public functions, and really should
appear in the documentation, but don't since they're private, or are
implementation details that are currently public.
Bring in the line-length police
auto merge of #11762 : alexcrichton/rust/guard_pages, r=alexcrichton

Rebasing of the previous PRs, I believe I've found the problems.
Refactor the build system for easily adding crates

Before this patch, if you wanted to add a crate to the build system you had to
change about 100 lines across 8 separate makefiles. This is highly error prone
and opaque to all but a few. This refactoring is targeted at consolidating this
effort so adding a new crate adds one line in one file in a way that everyone
can understand it.
syntax: Fix a missing closing code tag in docs.
Change private structs to have private fields by default

This was the original intention of the privacy of structs, and it was
erroneously implemented before. A pub struct will now have default-pub fields,
and a non-pub struct will have default-priv fields. This essentially brings
struct fields in line with enum variants in terms of inheriting visibility.

As usual, extraneous modifiers to visibility are disallowed depend on the case
that you're dealing with.

Closes #11522