get recursive copy to work
include the capn_segment as a header when allocating
remove read/write_float, use to/from_f32 instead
fix capn_setp to copy nulls correctly
switch back to capn_root

capn_root now returns a ptr that can be used with capn_getp/setp.
This replaces capn_new_root and capn_get_root.
add support for default values