Note new iterate syntax
- Renamed `~+` to `~mod+`; added `~mod-`, `~sat+` and `~sat-`.
- Removed `&^`.
- Renamed `[N] T` and `[] T` types to `array[N] T` and `slice T`.
-- Renamed `iterate.N` to `iterate[N]`; added step argument.
+- Renamed `iterate.U` to `iterate (step:S, unroll:U)`.
- Renamed `buf1`, `reader1`, etc to `io_buffer`, `io_reader`, etc.
- Renamed `u32`, `io_reader`, etc to `base.u32`, `base.io_reader`, etc.
- Added a `base.image_config` type.
