[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] Rewrite encoding tests to separate out to-be-generated data

Also format all fidl-related code with a 100 column limit.

Change-Id: I1b7b66ab3ac1a1b88dc1217d50904249a9539000