maint: update to unreleased stdlib-41 api changes.

* bin/zemacs, bin/zz: Use `std.require` instead of deprecated
`std.string.require_version`.
* lib/zile/file.lua, lib/zz/commands/buffer.lua: Use
`std.ripairs` instead of deprecated `std.table.ripairs`.
* lib/zemacs/eval.lua, lib/zile/term_curses: Use `std.memoize`
instead of deprecated `std.functional.memoize`.
* lib/zile/keycode.lua: Likewise.
(keycode): Return only the first result from memoized_keycode,
now that std.memoize returns multiple values properly.
* lib/zile/syntax.lua: Use `std.vector` instead of aborted
`std.array` object.
* README.md: Update.
Add a "unreleased Zile sometimes requires unreleased modules"
note.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>