This commit adds a new `RingIO` type which exposes the internal ring-buffer
code for general use in Python programs. It has the stream interface
making it similar to `StringIO` and `BytesIO`, except `RingIO` has a fixed
buffer size and is automatically safe when reads and writes are in
different threads or an IRQ.
This new type is enabled at the "extra features" ROM level.
Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>
