Implement len() for EnumSet.
Renamed Extendable to Extend

In order to upgrade, simply rename the Extendable trait to Extend in
your code

Part of #18424

[breaking-change]
Implements Extend for EnumSet and LruCache

Part of #18424
auto merge of #18740 : jbcrail/rust/implement-enum-set-len, r=alexcrichton

This commit adds the missing EnumSet method mentioned by @Gankro.

cc #18424
auto merge of #18475 : gamazeps/rust/toExtend, r=alexcrichton

Ensured that Extend & FromIterator are implemented for the libcollection.

Removed the fact that FromIterator had to be implemented in order to implement Extend, as it did not make sense for LruCache (it needs to be given a size and there are no Default for LruCache).

Changed the name from Extend to Extendable.

Part of #18424
Runtime removal: add private sys, sys_common modules

These modules will house the code that used to be part of the runtime system
in libnative. The `sys_common` module contains a few low-level but
cross-platform details. The `sys` module is set up using `#[cfg()]` to
include either a unix or windows implementation of a common API
surface. This API surface is *not* exported directly in `libstd`, but is
instead used to bulid `std::os` and `std::io`.

Ultimately, the low-level details in `sys` will be exposed in a
controlled way through a separate platform-specific surface, but that
setup is not part of this patch.
Remove signal handling.

Since signal handling was only implemented through librustuv, which is
now gone, there's no reason to even provide the API.

[breaking-change]
Runtime removal: refactor fs

This moves the filesystem implementation from libnative into the new
`sys` modules, refactoring along the way and hooking into `std::io::fs`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor pipes and networking

This patch continues the runtime removal by moving pipe and
networking-related code into `sys`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor helper threads

This patch continues the runtime removal by moving
libnative::io::helper_thread into sys::helper_signal and
sys_common::helper_thread

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor process

This patch continues the runtime removal by moving and refactoring the
process implementation into the new `sys` module.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor timer

This patch continues runtime removal by moving out timer-related code
into `sys`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor tty

This patch continues runtime removal by moving the tty implementations
into `sys`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: fully remove rtio

This patch cleans up the remnants of the runtime IO interface.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Remove somewhat bogus process-spawn-errno test (non-mac, non-windows only)