[fidl] Add fidl::Message data structure

The fidl::Message data structure holds both the byte and the handle part
of a FIDL message and knows how to encode and decode messages in place.
The fidl::Message also knows about the message header and its various
fields.

This CL also moves the C++-related headers into a cpp directory to make
it easier to see which parts of the library are related to the C++
bindings and which are useful from C.

Change-Id: I7031d1b3d9a76b911f7e9de9c3f5aa33a0f700f8

[fidl][coding] Implement fidl decoding

This defines the interface to fidl encoding and decoding. This
includes the definitions of fidl types like fidl_string_t, and the
declarations of fidl_encode and fidl_decode.

This also implements and tests fidl_decode.

TO-487 #done

Change-Id: I7a5490c6efdd9c4ea045a9a1851030441bc72299