[fidl] Remove fidl::Array

This is not used in Zircon, and we need to specialize equality for it so
that it correctly handles array of nullable structs.

Change-Id: Idff682a2a37faaf82d90c7a12f8a4dc27b7bb9b3

[fidl] Move public fidl headers to match naming convention

The fidl library provides headers intended for general use, so the
include path should be spelled <lib/fidl/foo.h> as per
https://fuchsia.googlesource.com/docs/+/master/development/languages/c-cpp/naming.md.
This moves the headers into the proper location and updates includes to
match.

Change-Id: I9db162c3fd40170d63730bc9b7079c7e7e950982

[fidl] Add fidl::Array type

Wrapping arrays in a struct makes them easier to deal with because they
act more like normal C++ types.

Change-Id: I5ca4d0a5e2494e13f6e7d51c5715d56dd7ed875d

[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] Add fidl::String

This patch starts to introduce the C++ FIDL types. These types are views
into the underlying C types that are used by the encode and decode
functions. For decoding, these views are ultimately on the underlying
message buffer, similar to the C types. For encoding, the underlying C
types are stored in a builder and manipulated through the C++ views.

Change-Id: I21b8e2be687c2cfed57d395898e31e3d16bc9961