Add a way to extend record types in the idl

This puts a generic extension function into the Rpc module. For
an example of usage, see the idl_test.ml function.

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Add .gitignore

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Add a Makefile rule to make the ml file generated from the idl

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>