refactor: adjust to different std.set membership syntax in v37.

Now that `std.set` is a `std.container` derived object, there
are no object methods.
* lib/zile/file.lua, lib/zile/keycode.lua, lib/zile/minibuf.lua,
lib/zile/term_minibuf.lua, lib/zmacs/zlisp/search.zl,
lib/zz/commands/search.lua: Use `set.member (s, e)` in place of
`s:member (e)`.

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