std: Add close_{read,write}() methods to I/O

Two new methods were added to TcpStream and UnixStream:

    fn close_read(&mut self) -> IoResult<()>;
    fn close_write(&mut self) -> IoResult<()>;

These two methods map to shutdown()'s behavior (the system call on unix),
closing the reading or writing half of a duplex stream. These methods are
primarily added to allow waking up a pending read in another task. By closing
the reading half of a connection, all pending readers will be woken up and will
return with EndOfFile. The close_write() method was added for symmetry with
close_read(), and I imagine that it will be quite useful at some point.

Implementation-wise, librustuv got the short end of the stick this time. The
native versions just delegate to the shutdown() syscall (easy). The uv versions
can leverage uv_shutdown() for tcp/unix streams, but only for closing the
writing half. Closing the reading half is done through some careful dancing to
wake up a pending reader.

As usual, windows likes to be different from unix. The windows implementation
uses shutdown() for sockets, but shutdown() is not available for named pipes.
Instead, CancelIoEx was used with same fancy synchronization to make sure
everyone knows what's up.

cc #11165
auto merge of #13751 : alexcrichton/rust/io-close-read, r=brson

Two new methods were added to TcpStream and UnixStream:

    fn close_read(&mut self) -> IoResult<()>;
    fn close_write(&mut self) -> IoResult<()>;

These two methods map to shutdown()'s behavior (the system call on unix),
closing the reading or writing half of a duplex stream. These methods are
primarily added to allow waking up a pending read in another task. By closing
the reading half of a connection, all pending readers will be woken up and will
return with EndOfFile. The close_write() method was added for symmetry with
close_read(), and I imagine that it will be quite useful at some point.

Implementation-wise, librustuv got the short end of the stick this time. The
native versions just delegate to the shutdown() syscall (easy). The uv versions
can leverage uv_shutdown() for tcp/unix streams, but only for closing the
writing half. Closing the reading half is done through some careful dancing to
wake up a pending reader.

As usual, windows likes to be different from unix. The windows implementation
uses shutdown() for sockets, but shutdown() is not available for named pipes.
Instead, CancelIoEx was used with same fancy synchronization to make sure
everyone knows what's up.

cc #11165
add back jemalloc to the tree

This adds a `std::rt::heap` module with a nice allocator API. It's a
step towards fixing #13094 and is a starting point for working on a
generic allocator trait.

The revision used for the jemalloc submodule is the stable 3.6.0 release.

Closes #11807
add an align parameter to exchange_malloc

Closes #13094
use jemalloc to implement Vec<T>
initial port of the exchange allocator to jemalloc

In stage0, all allocations are 8-byte aligned. Passing a size and
alignment to free is not yet implemented everywhere (0 size and 8 align
are used as placeholders). Fixing this is part of #13994.

Closes #13616
fix Vec<ZeroSizeType>
gitattributes: rm obsolete jemalloc entry