[memfs] Track and limit number of pages used in a memfs.

Creating/writing to files which exceeds the page limit
will fail with ZX_ERR_NO_SPACE.
After we switch from Ioctl to FIDL interface, memfs can
expose free/used space information.

ZX-2699 #done

Test: unit tests in QEMU.

Change-Id: If9f43aab3c5d6645e9405dade69df71698cfdf11

[sync] Apply C API rubric to libsync

Test: No behavior change
Change-Id: I30a413f7fb5d3f4e741e5204532b82e73595c086

[async] Change async_t -> async_dispatcher_t

Change name of dispatcher type from async_t to async_dispatcher_t
Keep typedef of async_t for compatibility

Change async_{get,set}_default() to async_{get,set}_default_dispatcher()
Again, keep compatible apis until other layers have been modified to use
new api.

Bug:ZX-2337
Test:CQ
Change-Id: If0b5d061fd114d60e23078dbb5f2a6ed9ed6f6cc

[memfs] Improve API readability

Minor tweaks to comply with API readability rubric.

Test: No behavior change
Change-Id: I9005f1569bec43ad54d291eb10d86fba17f45e9e

[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[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