BlobList: use recommended sorting primitives

[ahouene]

Merely scratching an itch here, but the docs for the sort.Sort function we're using to sort BlobList mention:

Note: in many situations, the newer slices.SortFunc function is more ergonomic and runs faster.

And indeed:

$ go test . -bench=BlobListSort -benchmem -count=30 | benchstat -
goos: linux
goarch: amd64
pkg: github.com/PowerDNS/simpleblob
cpu: 11th Gen Intel(R) Core(TM) i9-11950H @ 2.60GHz
                                │      -      │
                                │   sec/op    │
BlobListSort/sort.Sort-16         912.7n ± 1%
BlobListSort/slices.SortFunc-16   823.7n ± 2%
geomean                           867.0n

                                │      -       │
                                │     B/op     │
BlobListSort/sort.Sort-16         24.00 ± 0%
BlobListSort/slices.SortFunc-16   0.000 ± 0%
geomean                                      ¹
¹ summaries must be >0 to compute geomean

                                │      -       │
                                │  allocs/op   │
BlobListSort/sort.Sort-16         1.000 ± 0%
BlobListSort/slices.SortFunc-16   0.000 ± 0%
geomean                                      ¹
¹ summaries must be >0 to compute geomean

This PR thus changes to slices.SortFunc, and provides a convenience (BlobList).Sort method.