Golay codec optimisation for space and speed (esp. decoding)

The Golay23 codec in SiK/radio could use some optimisations. An
improved version is proposed in this commit. Changes to the upstream
version, with observations motivating those changes, are outlined below.

1. Code table width

The tables used for encoding and decoding are 32 bits wide, which is a
complete waste of ROM space. We are only interested in recovering the
12-bit payload data -- we don't really care about whether errors that
fall into the parity symbol part of the codeword get corrected or not.
We throw that part away anyway.

So only 11 bits of the encoder table are significant to us -- 11 bits
is the width of the syndrome that, combined with our 12-bit payload,
will form the 23 bits of the Golay23 codeword. For easy storage, we
store these values in a 16 bit wide table. The table still contains
4096 values, but requires half the storage space.

Naturally, the decoder table also can be restricted to the meaningful
part. In this case, the table contains the error correction lookup
values of which 12 bits are useful -- the width of our payload we wish
to correct. For easy storage, we store these also in a 16 bit wide
table. The table still contains 2048 values, but requires half the
storage space.

2. Decoding algorithm

That was trivial, you might say. Now comes the interesting part. The
decoding of Golay23 codewords can be done in much less work compared
to the existing implementation. Specifically, the whole syndrome
calculation function is redundant. Why calculate the syndrome on the
spot each time, when it is already stored in a table?

Realise that the encoder table is nothing but precomputed values of
the syndrome for all possible payload values. The encoding operation
is in fact nothing more but a lookup to obtain the syndrome value
corresponding to the payload at hand, and appending it to obtain the
encoded codeword. The exact same operation is usable to obtain the
syndrome when decoding. The difference, of course, is that the
received payload (the part of the received codeword that *is* the
payload) may contain some bit errors. Nevertheless, we look it up in
the encoder table to obtain the syndrome corresponding to that
payload, were it the real payload that was sent (and if there is no
error, it is).

Since the operations over the code space are linear, XORing this
looked-up syndrome with the received parity will yield the *real*
received syndrome. The same as the expensive syndrome calculator
function would have yielded. That is, we traded a function call with a
loop over individual bits for a cheap table lookup and an XOR.

3. Re-arrange the bytes

Since the Golay codec in the SiK firmware always works in multiples of
3 bytes for payload (and mutiples of 6 bytes for encoded data), it is
natural that we can rearrange the internal order of bytes within these
3-packs and 6-packs to shave off some bit-twiddling operations.

Specifically, the first 3 bytes of an encoded 6-pack are the same as
the 3 bytes input to golay_encode(). The next three bytes contain the
parity information:
- the first byte is the low 8 bits of parity of the first codeword;
- the second byte's high nibble is the most significant 4 bits of
  the parity of the first codeword;
- the second byte's low nibble is the most significant 4 bits of
  the parity of the second codeword;
- the third byte is the low 8 bits of parity of the third codeword.

I don't have access to the real hardware so I cannot measure the
real-time improvement these changes yield. I estimate the time
required to run golay_decode24() to be about one third of the
original.

I did verify the codec implementation driving it via a host program
(compiled with GCC on Linux) acting as a testbench: generating random
payload, encoding, applying errors, decoding and verifying that the
original payload was recovered. In case of interest, I shall be happy
to supply this testbench application.