reduce RSSubCode allocs

This reduces allocs by about 50%:

master:
BenchmarkRSSubCodeRecover-4     247.51 MB/s   241153 B/op    1280 allocs/op

this branch:
BenchmarkRSSubCodeRecover-4     277.40 MB/s   138240 B/op     617 allocs/op

Unfortunately we cannot do much better than this without modifying the reedsolomon package. reedsolomon allocates some temporary buffers during decoding and passes them to different goroutines, forcing them to be heap-allocated. This isn't a big deal when you're decoding 4MB shards, but because the subcode calls Recover on every segment, we end up doing hundreds of thousands of allocations per sector.

I was able to achieve zero-alloc encoding and decoding in us by forking reedsolomon and removing the parallelism. Such a change is unlikely to be accepted upstream, though, because larger decodes benefit from parallelism; most people aren't doing subcodes. So vendoring reedsolomon may be our best bet here.