Update Option interface

- Added `String()`, `Type()`, and `Value()` methods to the interface to
  aid with debugging and error handling.

scheduler_test.go

@@ -5,7 +5,6 @@
 package asynq
 
 import (
-	"fmt"
 	"testing"
 	"time"
 
@@ -77,37 +76,3 @@ func TestScheduler(t *testing.T) {
 		}
 	}
 }
-
-func TestStringifyOptions(t *testing.T) {
-	now := time.Now()
-	oneHourFromNow := now.Add(1 * time.Hour)
-	twoHoursFromNow := now.Add(2 * time.Hour)
-	tests := []struct {
-		opts []Option
-		want string
-	}{
-		{
-			opts: []Option{MaxRetry(10)},
-			want: "MaxRetry(10)",
-		},
-		{
-			opts: []Option{Queue("custom"), Timeout(1 * time.Minute)},
-			want: `Queue("custom"), Timeout(1m0s)`,
-		},
-		{
-			opts: []Option{ProcessAt(oneHourFromNow), Deadline(twoHoursFromNow)},
-			want: fmt.Sprintf("ProcessAt(%v), Deadline(%v)", oneHourFromNow, twoHoursFromNow),
-		},
-		{
-			opts: []Option{ProcessIn(30 * time.Minute), Unique(1 * time.Hour)},
-			want: "ProcessIn(30m0s), Unique(1h0m0s)",
-		},
-	}
-
-	for _, tc := range tests {
-		got := stringifyOptions(tc.opts)
-		if got != tc.want {
-			t.Errorf("got %v, want %v", got, tc.want)
-		}
-	}
-}