Remove use of ppx
Remove use of ppx

Previously we used a trivial amount of ppx for parsing and printing some
trivial vmnet protocol message types (some of which consisted of only
a single uint8_t). This patch removes the syntax extension to simplify
and speed up the build.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit