[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] Improve cpp ergonomics

These tweaks are useful form the upper layers.

Change-Id: Id3ddace84f55ce4b054bcb6844b5144380b30508

[fidl] Factor MessageBuffer out of MessageBuilder

Turns out the memory management of MessageBuffer is useful on the
receiving side as well.

Change-Id: I37d12c489c16f872e9e12afdd675043f1513aa63

[fidl] Add MessageBuilder, which owns the message memory

MessageBuilder is a utility class that makes it easy to build up
messages. Clients that wish to manage memory more precisely should use
Builder and Message separately.

Change-Id: Ia2f664a5d3404bcb224bf58d473dc2edbfef2edb