`^D` moves the cursor down half a page
Rename to b/scroll-down-half-page
`^D` won't scroll past end-of-file
`^D` won't move the cursor past end-of-file
`^D` won't scroll when file is shorter than screen
`^D` won't move cursor past end-of-file when file is shorter than screen
`^U` on the first line beeps
`^U` scrolls up a half page
`^U` does not scroll to before the first line of file
`^U` does not move cursor before beginning of file
Rename b/lines to b/line-count
Unify scroll-{up,down}-half-page
Refactor b/scroll-half-page
Add :pre for move-to-line
Rename scroll-half-page -> move-and-scroll-half-page
Rename with-cursor -> move-cursor
Compose key handler functions up front
Introduce defhandler macro
Extract avi.keymap
Make handler names stable