Use a code generator to call functions from wire format

This change removes the dependency on libffi and the header-dependency on
libwayland. (`wayland-xml` is still needed, of course.)

A python script is used to generate a series of 'call_*' functions which
parse the wire format and invoke matching `do_*` functions to be defined
by the user. With lto the `call_*` and `do_*` parts can often be combined,
reducing their effective cost to almost zero.

The script also generates more optimized data to check sizes and generate
new objects.