Avoid re-initializing decoders

[perlinm]

At the moment, qldpc.decoder.decode* methods accept both a matrix and a syndrome vector. However, a significant fraction of decoding time is spent initializing a decoder out of the matrix alone. It would be nice to avoid re-initializing the decoder when possible.

There may not be a way to avoid having to repeatedly initialize a decoder for code distance estimation, because this task involves constructing a new "effective" parity check matrix at every call of the decoder. However, it would be reasonable for each AbstractCode instance to recycle the same decoder for "ordinary" decoding tasks.