Handler: add NewMapped, AddMapped

The Handler now supports serving multiple packfiles, possibly with
different prefixes. Precedence is given by order in which packfiles are
registered.

handler_test.go

@@ -0,0 +1,49 @@
+package htpack
+
+import (
+	"bytes"
+	"io"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"src.lwithers.me.uk/go/htpack/packed"
+)
+
+func TestHandler(t *testing.T) {
+	mapped, err := packed.MapAndLoad("packed/testdata/helloworld.pack")
+	if err != nil {
+		t.Fatal("loading test packfile:", err)
+	}
+	defer mapped.Close()
+
+	orig, err := os.ReadFile("packed/testdata/helloworld.txt")
+	if err != nil {
+		t.Fatal("loading test textfile:", err)
+	}
+
+	h := NewMapped("", mapped)
+	h.AddMapped("/foo", mapped)
+
+	testsv := httptest.NewServer(h)
+	defer testsv.Close()
+	cl := testsv.Client()
+	url := testsv.URL + "/helloworld.txt"
+
+	resp, err := cl.Get(url)
+	if err != nil {
+		t.Fatal("executing GET request:", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal("reading response body:", err)
+	}
+
+	if !bytes.Equal(body, orig) {
+		t.Error("difference in response (- got, + expected):")
+		t.Error(cmp.Diff(string(body), string(orig)))
+	}
+}