internal/pkg: decompress xz streams

This uses a port of the public domain xz implementation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/xz/doc.go

@@ -0,0 +1,29 @@
+// Package xz implements XZ decompression natively in Go.
+//
+// # Usage
+//
+// For ease of use, this package is designed to have a similar API to
+// compress/gzip. See the examples for further details.
+//
+// # Implementation
+//
+// This package is a translation from C to Go of XZ Embedded
+// (http://tukaani.org/xz/embedded.html) with enhancements made so as
+// to implement all mandatory and optional parts of the XZ file format
+// specification v1.0.4. It supports all filters and block check
+// types, supports multiple streams, and performs index verification
+// using SHA-256 as recommended by the specification.
+//
+// # Speed
+//
+// On the author's Intel Ivybridge i5, decompression speed is about
+// half that of the standard XZ Utils (tested with a recent linux
+// kernel tarball).
+//
+// # Thanks
+//
+// Thanks are due to Lasse Collin and Igor Pavlov, the authors of XZ
+// Embedded, on whose code package xz is based. It would not exist
+// without their decision to allow others to modify and reuse their
+// code.
+package xz