[memfs] Implement QueryFilesystem FIDL interface.

Allows querying disk pressure.

Test: Unit tested on QEMU.
Change-Id: I512f941b33e215544b84f2b039c30b6c78a4cbde

[fs] Implement Describe for C++ file systems

This CL implements the Describe method on Node.

Test: memfs-test.test_fidl_basic
Change-Id: I07be854beb04a7d42fe3c08a1a9aac85c361641c

[memfs] Split memfs into C / C++ libraries

For clients which utilize memfs to create
temporary filesystems, export a C ABI shared
library.

Internally, this C library uses a C++ library,
which is exported as source.

Change-Id: I73b95a2a2b8c4870d1b07f5f2f2525f4b0cd4e9f

[memfs] Implement safe filesystem teardown for memfs

ZX-2032 #done
ZX-1576 #comment In Progress

Change-Id: Id5a30b635883c9322ad4838cafdcc770726c91eb

[async] Move async-loop implementation to its own directory.

People keep getting confused by the relationship between async-loop
and the rest of the library. So let's fix it.

This change includes compatibility includes for loop.h to be removed
after upper layers are fixed to use the new paths.

Change-Id: I95c57be533fbd91468275bb2470890a4f10101eb

[async] Separate fbl dependency from async_t

The C++ FIDL support library in Garnet wants to be able to use async_t
without linking against libfbl. Previously, the C declarations needed
create an async_wait_t were in the same header as code that depended on
fbl, which meant C++ clients of async_wait_t were forced to depend on
fbl.

After this change, the C++ wrappers for libasync interfaces are
separated from the core C declarations.

Change-Id: Ic172911a9dc775d5b5e540a33ce87d2dc218a3d9

[memfs] Add C API for creating /tmp filesystems

Change-Id: I6144c4fac5b6d9894299e90653642949e3747842