Increase sector root performance

[n8maninger]

Before

goos: linux
goarch: amd64
pkg: go.sia.tech/core/internal/blake2b
cpu: AMD EPYC 7B13 64-Core Processor                
BenchmarkBLAKE2b/SumLeaves-128           4167430               290.8 ns/op       880.38 MB/s           0 B/op          0 allocs/op
BenchmarkBLAKE2b/SumNodes-128            3793945               285.3 ns/op       897.44 MB/s           0 B/op          0 allocs/op
PASS
ok      go.sia.tech/core/internal/blake2b       4.610s

After

goos: linux
goarch: amd64
pkg: go.sia.tech/core/internal/blake2b
cpu: AMD EPYC 7B13 64-Core Processor                
BenchmarkBLAKE2b/SumLeaves-128           6697410               176.7 ns/op      1448.80 MB/s           0 B/op          0 allocs/op
BenchmarkBLAKE2b/SumNodes-128            5853004               181.6 ns/op      1409.40 MB/s           0 B/op          0 allocs/op
PASS
ok      go.sia.tech/core/internal/blake2b       4.376s

[lukechampine]

wild. Thanks!