k12: improve API and support multithreaded computation

Use options style API, so that the common case is very simple:

    h := k12.NewDraft10()

but we can provide options elegantly:

    h := k12.NewDraft10(
        WithContext([]byte("some context")),
        WithWorkers(runtime.NumCPU()),
    )

Allows multithreaded computation with the WithWorkers() option.
On M2 Pro scales well with a few workers, but isn't able to
utilize all cores effectively.

    BenchmarkK12_100B-12          	 5223334	       228.8 ns/op	 437.07 MB/s
    BenchmarkK12_10K-12           	  105744	     11183 ns/op	 894.21 MB/s
    BenchmarkK12_100K-12          	   27141	     44364 ns/op	2254.06 MB/s
    BenchmarkK12_3M-12            	    1172	   1010401 ns/op	3243.07 MB/s
    BenchmarkK12_32M-12           	     100	  10033730 ns/op	3265.78 MB/s
    BenchmarkK12_327M-12          	      12	  98888208 ns/op	3313.64 MB/s
    BenchmarkK12_3276M-12         	       2	 991111938 ns/op	3306.19 MB/s
    BenchmarkK12x2_32M-12         	     183	   6510423 ns/op	5033.16 MB/s
    BenchmarkK12x2_327M-12        	      18	  63622058 ns/op	5150.41 MB/s
    BenchmarkK12x2_3276M-12       	       2	 632823584 ns/op	5178.06 MB/s
    BenchmarkK12x4_32M-12         	     364	   3300120 ns/op	9929.34 MB/s
    BenchmarkK12x4_327M-12        	      39	  29477854 ns/op	11116.14 MB/s
    BenchmarkK12x4_3276M-12       	       2	 581819167 ns/op	11263.98 MB/s
    BenchmarkK12x8_32M-12         	     520	   2301923 ns/op	14235.05 MB/s
    BenchmarkK12x8_327M-12        	      76	  15590312 ns/op	21018.18 MB/s
    BenchmarkK12x8_3276M-12       	       4	 296590469 ns/op	22096.46 MB/s
    BenchmarkK12xCPUs_32M-12      	     472	   2526827 ns/op	12968.04 MB/s
    BenchmarkK12xCPUs_327M-12     	      78	  15139957 ns/op	21643.39 MB/s
    BenchmarkK12xCPUs_3276M-12    	       4	 280958114 ns/op	23325.90 MB/s

We only reach 23GB/s (at 12x) instead of the lower bound of 33GB/s
expected with 10 performance cores.

Adds {Max,Next}WriteSize to suggest the caller how big to choose
their Write() calls.

Adds a test to the CI, to check the k12 package with the data race
detector on.

.github/workflows/ci-actions.yml

@@ -41,6 +41,8 @@ jobs:
         run: go build -v ./...
       - name: Testing
         run: go test -v -count=1 ./...
+      - name: Testing with data race detector
+        run: go test -v -count=1 ./xof/k12
   exotic_job:
     name: Go-${{matrix.CFG[2]}}/${{matrix.CFG[0]}}
     needs: [amd64_job]