packed: add ability to load from memory, add MapAndLoad

The ability to load a packfile from memory means that it becomes easy to
use the "embed" package to embed a small packfile.

The refactor to allow loading from memory also makes it easy to add a
MapAndLoad implementation, which means the file only needs to be opened
/ mmap()ed once.

packed/load_test.go

@@ -0,0 +1,39 @@
+package packed
+
+import (
+	"bytes"
+	"os"
+	"testing"
+)
+
+func TestMapAndLoad(t *testing.T) {
+	rawPack, err := os.ReadFile("testdata/helloworld.pack")
+	if err != nil {
+		t.Fatal(err)
+	}
+	rawText, err := os.ReadFile("testdata/helloworld.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mapped, err := MapAndLoad("testdata/helloworld.pack")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer mapped.Close()
+
+	if !bytes.Equal(mapped.Data, rawPack) {
+		t.Error("mapped data does not match raw file")
+	}
+
+	file := mapped.Directory.Files["/helloworld.txt"]
+	if file == nil {
+		t.Fatal("directory does not contain /helloworld.txt")
+	}
+
+	start := file.Uncompressed.Offset
+	end := start + file.Uncompressed.Length
+	if !bytes.Equal(rawText, mapped.Data[start:end]) {
+		t.Error("uncompressed file does not match testdata/helloworld.txt")
+	}
+}