Switch to SIMD-optimized SHA256 implementation

The SHA256 implementation provided by the github.com/minio/sha256-simd
package is measurably faster than the standard library's crypto/sha256
package:

```
name                          old speed     new speed      delta
StreamWriteTo/tar/1.0MiB-4    394MB/s ± 6%  1024MB/s ± 6%  +160.10%  (p=0.000 n=9+10)
StreamWriteTo/tar/8.0MiB-4    452MB/s ± 2%  1444MB/s ± 4%  +219.48%  (p=0.000 n=9+9)
StreamWriteTo/tar/16.0MiB-4   462MB/s ± 1%  1470MB/s ± 1%  +218.07%  (p=0.000 n=10+9)
StreamWriteTo/tar/128.0MiB-4  464MB/s ± 0%  1444MB/s ± 3%  +211.36%  (p=0.000 n=8+10)
StreamWriteTo/tgz/1.0MiB-4    310MB/s ± 9%   594MB/s ±13%   +91.77%  (p=0.000 n=10+8)
StreamWriteTo/tgz/8.0MiB-4    403MB/s ± 2%  1012MB/s ± 4%  +150.86%  (p=0.000 n=10+9)
StreamWriteTo/tgz/16.0MiB-4   408MB/s ± 1%  1069MB/s ± 4%  +161.80%  (p=0.000 n=9+10)
StreamWriteTo/tgz/128.0MiB-4  418MB/s ± 1%  1101MB/s ± 6%  +163.57%  (p=0.000 n=10+10)
```

Benchmark command:

```
go test -cpu=4 -benchtime=1s -count=10 -bench=. \
  ./internal/snapshotstream
```

Results compared using golang.org/x/perf/cmd/benchstat.

Author: hansmi

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/gorilla/handlers v1.5.1
 	github.com/gorilla/mux v1.8.0
 	github.com/klauspost/compress v1.15.12
+	github.com/minio/sha256-simd v1.0.0
 	github.com/prometheus/client_golang v1.14.0
 	github.com/prometheus/common v0.37.0
 	go.uber.org/multierr v1.8.0
@@ -19,6 +20,7 @@ require (
 	github.com/felixge/httpsnoop v1.0.3 // indirect
 	github.com/golang/protobuf v1.5.2 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
+	github.com/klauspost/cpuid/v2 v2.0.4 // indirect
 	github.com/matttproud/golang_protobuf_extensions v1.0.4 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect

go.sum

@@ -151,6 +151,8 @@ github.com/julienschmidt/httprouter v1.3.0/go.mod h1:JR6WtHb+2LUe8TCKY3cZOxFyyO8
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/klauspost/compress v1.15.12 h1:YClS/PImqYbn+UILDnqxQCZ3RehC9N318SU3kElDUEM=
 github.com/klauspost/compress v1.15.12/go.mod h1:QPwzmACJjUTFsnSHH934V6woptycfrDDJnH7hvFVbGM=
+github.com/klauspost/cpuid/v2 v2.0.4 h1:g0I61F2K2DjRHz1cnxlkNSBIaePVoJIjjnHui8QHbiw=
+github.com/klauspost/cpuid/v2 v2.0.4/go.mod h1:FInQzS24/EEf25PyTYn52gqo7WaD8xa0213Md/qVLRg=
 github.com/konsorten/go-windows-terminal-sequences v1.0.1/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
 github.com/konsorten/go-windows-terminal-sequences v1.0.3/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
 github.com/kr/logfmt v0.0.0-20140226030751-b84e30acd515/go.mod h1:+0opPa2QZZtGFBFZlji/RkVcI2GknAs/DXo4wKdlNEc=
@@ -160,6 +162,8 @@ github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/matttproud/golang_protobuf_extensions v1.0.1/go.mod h1:D8He9yQNgCq6Z5Ld7szi9bcBfOoFv/3dc6xSMkL2PC0=
 github.com/matttproud/golang_protobuf_extensions v1.0.4 h1:mmDVorXM7PCGKw94cs5zkfA9PSy5pEvNWRP0ET0TIVo=
 github.com/matttproud/golang_protobuf_extensions v1.0.4/go.mod h1:BSXmuO+STAnVfrANrmjBb36TMTDstsz7MSK+HVaYKv4=
+github.com/minio/sha256-simd v1.0.0 h1:v1ta+49hkWZyvaKwrQB8elexRqm6Y0aMLjCNsrYxo6g=
+github.com/minio/sha256-simd v1.0.0/go.mod h1:OuYzVNI5vcoYIAmbIvHPl3N3jUzVedXbKy5RFepssQM=
 github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
 github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd h1:TRLaZ9cD/w8PVh93nsPXa1VrQ6jlwL5oN8l14QlcNfg=
 github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=

internal/snapshotstream/stream.go

@@ -1,7 +1,6 @@
 package snapshotstream
 
 import (
-	"crypto/sha256"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -15,6 +14,7 @@ import (
 
 	"github.com/hansmi/prombackup/api"
 	"github.com/klauspost/compress/gzip"
+	"github.com/minio/sha256-simd"
 	"go.uber.org/multierr"
 )