feat: implement new faster sort algorithm for Go 1.18

This implements the pdqsort algorithm for Go 1.18 and later. The new
algorithm is ~10x faster than `sort.Sort` and was added to the
`slices` package in Go 1.20.

The algorithm is backported from the stdlib and is used in this
package to provide a faster median cut algorithm.

Author: aymanbagabas

go.mod

@@ -1 +1,3 @@
-module "github.com/soniakeys/quant"
+module github.com/soniakeys/quant
+
+go 1.18