alloc-ified many functions.

Updated README

src/options.zig

@@ -5,6 +5,8 @@ const Writer = std.Io.Writer;
 
 const ExtractionOptions = @This();
 
+/// The number of threads used for extraction. 0 implies single threaded.
+threads: usize = 1,
 /// Don't set the file's owner & permissions after extraction
 ignore_permissions: bool = false,
 /// Don't set xattr values. Currently xattrs are never set anyway.
@@ -16,10 +18,16 @@ verbose: bool = false,
 /// Where to print verbose log.
 verbose_writer: ?*Writer = null,
 
-pub const Default: ExtractionOptions = .{};
-pub fn VerboseDefault(wrt: *Writer) ExtractionOptions {
+pub const SingleThreadedDefault: ExtractionOptions = .{};
+pub fn Default() !ExtractionOptions {
+    return .{
+        .threads = try std.Thread.getCpuCount(),
+    };
+}
+pub fn VerboseDefault(wrt: *Writer) !ExtractionOptions {
     return .{
         .verbose = true,
         .verbose_writer = wrt,
+        .threads = try std.Thread.getCpuCount(),
     };
 }