encoder/filename: Add SCSU as tables

Instead of only adding SCSU, add it as an existing table.

Allow direct SCSU and add a, perhaps, reasonable table as well.

Add byte interfaces that doesn't base64 encode the URL as well with `EncodeBytes` and `DecodeBytes`.

Fuzz tested and decode tests added.

lib/encoder/filename/decode_test.go

@@ -0,0 +1,92 @@
+package filename
+
+import "testing"
+
+func TestDecode(t *testing.T) {
+	tests := []struct {
+		name    string
+		encoded string
+		want    string
+		wantErr bool
+	}{
+		{
+			name:    "unicode-1",
+			encoded: "8D5V3MESVd-WEF7WuqaOvpKUWtYGEyw5UDQ==",
+			want:    "長い長いＵＮＩＣＯＤＥファイル名",
+			wantErr: false,
+		},
+		{
+			name:    "unicode-2",
+			encoded: "8GyHV1N7u2OEg4ufQ3eHQ3Ngg6N3X0CDg4-HX0NXU2tg=",
+			want:    "ვეპხის ტყაოსანი შოთა რუსთაველი",
+			wantErr: false,
+		},
+		{
+			name:    "unicode-3",
+			encoded: "7LpehMXOrWe7mcT_lpf2MN1Nmgu55jpXHLavZcXJb2UTJ-UmGU15iznkD",
+			want:    "Sønderjysk: Æ ka æe glass uhen at det go mæ naue.,",
+			wantErr: false,
+		},
+		{
+			name:    "unicode-4",
+			encoded: "7TCSRm0liJDR0ulpBq4Lla_XB2mWdLFMEs8wEQKHAGa8FRr333ntJ6Ww6_f__N5VKeYM=",
+			want:    "Hello------world     時危兵甲滿天涯，載道流離起怨咨.bin",
+		},
+		{
+			name:    "plain-1",
+			encoded: "BzGQYxqHBA6ljTsir80gUM5Y=",
+			want:    "-Duplican99E8ZI4___9_",
+			wantErr: false,
+		},
+		{
+			name:    "hex-1",
+			encoded: "D_--tHZROQpqqJ9PafqNa6STF",
+			want:    "13646871dfabbs43323564654bbefff",
+			wantErr: false,
+		},
+		{
+			name:    "base64-1",
+			encoded: "FMpABB9Ef0KP8OrVxjnE3LzUePuLZi8pPg7eW8bgyW2d3Ucckf4rlE0mkAvlILVpOmF3L-rFbmNrpUO2HQFlF4SCMPVPeCEX6LeOg5JVpUVCXV1WSazD9vSpr",
+			want:    "UxAYiB0FNTTkXRw9P8hwq-WmN7tYwbe-sFw8C3snDRG1d-yjrdOUVZQyLdtkJ8tuvhBSnuBiLjVieCAroWEZDIO4Hb_rKgdzPjMqFE7inwHJ2isF==",
+			wantErr: false,
+		},
+		{
+			name:    "custom-1",
+			encoded: "-BeADJCoG_________________xc=",
+			want:    "Uaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+			wantErr: false,
+		},
+		{
+			name:    "rle-1",
+			encoded: "9a2E=",
+			want:    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+			wantErr: false,
+		},
+		{
+			name:    "regular-1",
+			encoded: "BeSSrnzj0j3OXyR9K81M=",
+			want:    "regular-filename.txt",
+			wantErr: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := Decode(tt.encoded)
+			if (err != nil) != tt.wantErr {
+				if tt.encoded == "" && tt.want != "" {
+					proposed := Encode(tt.want)
+					table := decodeMap[proposed[0]] - 1
+					t.Errorf("No encoded value, try '%s', table is %d", proposed, table)
+					return
+				}
+				t.Errorf("Decode() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if got != tt.want {
+				t.Errorf("Decode() got = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}