std.tar: support pax headers and gnulong_{name,link}

tar.zig:
* add HeaderIterator() type and convert pipeToFileSystem() to use it.
* add tests parseNumeric and parsePaxTime ported from
  https://go.dev/src/archive/tar/strconv_test.go.
* add initial support for options.executable_bit_only.
* initial windows support:
  * skip symlinks which require admin rights
  * workaround file.updateTimes() panic by truncating file times

lib/std/compress/tar/testdata/
* copy a subset of tar files from
  https://go.dev/src/archive/tar/testdata

reader_test.zig:
* validate headers against files from testdata/. a port of
  https://go.dev/src/archive/tar/reader_test.go.

reader_decompress.zig:
* runs tar.pipeToFileSystem() on valid testdata/ files.

Author: travisstaloch

lib/std/compress.zig

@@ -46,4 +46,5 @@ test {
     _ = xz;
     _ = zlib;
     _ = zstd;
+    _ = @import("compress/tar.zig");
 }

lib/std/compress/tar.zig

@@ -0,0 +1,4 @@
+test "std.tar" {
+    _ = @import("tar/reader_test.zig");
+    _ = @import("tar/test_decompress.zig");
+}

lib/std/compress/tar/reader_test.zig

@@ -0,0 +1,32 @@
+const std = @import("std");
+const mem = std.mem;
+const tar = std.tar;
+const testing = std.testing;
+
+test {
+    const testdata_path = "lib/std/compress/tar/testdata";
+    var testdata_dir = try std.fs.cwd().openIterableDir(testdata_path, .{});
+    var iter = testdata_dir.iterate();
+    var cache_dir = testing.tmpDir(.{});
+    defer cache_dir.cleanup();
+    const files_to_skip = [_][]const u8{
+        "neg-size.tar",
+        "pax-nul-xattrs.tar",
+        "pax-nul-path.tar",
+    };
+
+    entries: while (try iter.next()) |entry| {
+        if (entry.kind == .File) {
+            for (files_to_skip) |sf|
+                if (mem.eql(u8, sf, entry.name)) continue :entries;
+        } else continue;
+
+        const handle = try testdata_dir.dir.openFile(entry.name, .{});
+        defer handle.close();
+        const output_dir = try cache_dir.dir.makeOpenPath(entry.name, .{});
+        // Untar the archive using the decompresser
+        try tar.pipeToFileSystem(output_dir, handle.reader(), .{ .mode_mode = .ignore });
+        try output_dir.deleteTree(entry.name);
+        try tar.pipeToFileSystem(output_dir, handle.reader(), .{ .mode_mode = .executable_bit_only });
+    }
+}