pkg/nanors: add reed solomon codec implementation

[benpicco]

Contribution description

nanors provides fast reed solomon codes in GF2^8.
On Cortex-M4 the implementation (rs.o) only consumes a little less than 2k ROM.

Testing procedure

A test application is provided in tests/pkg_nanors.
It generates random data, parity shards and then deletes random shards and attempts to reconstruct them.

Issues/PRs references

[kaspar030]

Nice, stack usage is quite low:

        pid | name                 | state    Q | pri | stack  ( used) ( free) | base addr  | current     
          - | isr_stack            | -        - |   - |    512 (  164) (  348) | 0x20000000 | 0x200001c8
          1 | main                 | running  Q |   7 |   1536 (  504) ( 1032) | 0x20000f98 | 0x2000151c 
            | SUM                  |            |     |   2048 (  668) ( 1380)

(on nrf52840dk, probably the printf of ps used more)

[kaspar030]

ACK. Nice library!

[kaspar030]

Apart from the mips type mismatch (how annoying, didn't we deprecate mips...), nucleo-l011k4 arduino-duemilanove stk3200 arduino-uno arduino-nano nucleo-f031k6 atmega328p-xplained-mini arduino-leonardo samd10-xmini stm32f030f4-demo atmega328p are rom overflows.