[async] Change async_t -> async_dispatcher_t

This removes the compatibility wrappers once all the higher layers have
been migrated.

Bug:ZX-2337
Test:CQ
Change-Id: If8b59c157bbfadc63b426e679bce641ee7a7abd0

[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

[async] Move async headers to match naming rules

The naming rules for C/C++ header files that are extensively used but
not part of the 'system headers' or 'global headers' as defined by
https://fuchsia.googlesource.com/docs/+/master/development/languages/c-cpp/naming.md#include-paths
states that the include lines for headers in the 'async' library should
be spelled like this:

<lib/async/dispatcher.h>

This moves the header files from system/ulib/async/include/async/dispatcher.h
to system/ulib/async/include/lib/async/dispatcher.h and updates
includes to match.

Change-Id: I85de9f17d67605d59df763be54715a1f2aa9ed77

[async] Split async.h into multiple headers and use mxtl/macros.h.

Change-Id: I00d6a2479b0ebee467c73ac16f7c1c2533d9379c

[async] Split libasync into three parts.

Previously we built libasync as a shared library but that's not
quite correct. The client and message loop implementations should
be static libraries. It's only the default dispatcher functions
that need to be shared.

So we split the library into three parts: libasync.a, libasync-loop.a,
and libasync-default.so.

This also saves the linker some trouble since many clients of libasync
only need libasync.a and not the other parts.

Also improved the docs in a few areas to ensure we cover the C++ API
more thoroughly.

Change-Id: I98702dab4dfc21cbce87b8acddfc046a449410e6