Read an xz release in Go, so the build seat needs no xz tool (hq issue 345)

Betterbird's pinned .tar.xz failed on the build seat with "this build seat has
no xz to read it": the seat's image is the catalogue's build-agent recipe,
which installs git and docker-cli alone, and xz had been added only to
cmd/mesh-builder/Dockerfile, which the seat does not build from. The stream
is now read with github.com/ulikunitz/xz (vendored), its dictionary capped
at xz's own largest preset, and what it unpacks to bounded as before; the
xz line in the unused Dockerfile is taken back out.

vendor/github.com/ulikunitz/xz/lzma/bitops.go

@@ -0,0 +1,47 @@
+// Copyright 2014-2022 Ulrich Kunitz. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package lzma
+
+/* Naming conventions follows the CodeReviewComments in the Go Wiki. */
+
+// ntz32Const is used by the functions NTZ and NLZ.
+const ntz32Const = 0x04d7651f
+
+// ntz32Table is a helper table for de Bruijn algorithm by Danny Dubé.
+// See Henry S. Warren, Jr. "Hacker's Delight" section 5-1 figure 5-26.
+var ntz32Table = [32]int8{
+	0, 1, 2, 24, 3, 19, 6, 25,
+	22, 4, 20, 10, 16, 7, 12, 26,
+	31, 23, 18, 5, 21, 9, 15, 11,
+	30, 17, 8, 14, 29, 13, 28, 27,
+}
+
+/*
+// ntz32 computes the number of trailing zeros for an unsigned 32-bit integer.
+func ntz32(x uint32) int {
+	if x == 0 {
+		return 32
+	}
+	x = (x & -x) * ntz32Const
+	return int(ntz32Table[x>>27])
+}
+*/
+
+// nlz32 computes the number of leading zeros for an unsigned 32-bit integer.
+func nlz32(x uint32) int {
+	// Smear left most bit to the right
+	x |= x >> 1
+	x |= x >> 2
+	x |= x >> 4
+	x |= x >> 8
+	x |= x >> 16
+	// Use ntz mechanism to calculate nlz.
+	x++
+	if x == 0 {
+		return 0
+	}
+	x *= ntz32Const
+	return 32 - int(ntz32Table[x>>27])
+}