v3.9.0

• NcPlane.contents now returns NcResult.
• reexport most ffi functions generate by bindgen.
• make libc an optional dependency and do not require it for rstring_free macro, and several methods.
• remove macros sleep, nc_render_sleep, pile_render_sleep, visual_render_sleep.

Full Changelog: v3.8.0...v3.9.0

(compatible with notcurses v3.0.9)