Makes `cursive` and `pancurses` compile with v6 of `ncurses-rs`

This was originally made in PR https://github.com/jeaye/ncurses-rs/pull/218 but now it's split off[^1] for easier reviewing and merging, because that PR also has a ton of build improvements too and it's too difficult to review. With this PR, cursive and pancurses will compile afterwards but only if they also accept their own PRs which are https://github.com/gyscos/cursive/pull/778 respectively https://github.com/ihalila/pancurses/pull/93 Note, perhaps obvious to me, that currently `cursive` and `pancurses` cannot compile (without errors) with version `6.0.0` of `ncurses-rs` due to regressions that v6 introduced and also some incompatible changes, both of which this PR addresses, so they're stuck using version `5.101.0` [^1]: technically that PR still contains everything this PR, so merging that one doesn't require this one as well. Ideally, that one would be merged instead of this one, but hey, the option to not merge that one(at all, ever) is now available, if we only care about making cursive&pancurses use our version 6 as soon as possible(well, 2 months late anyway :D)