Routing: Reduce peak memory usage (#5488)

https://github.com/XTLS/Xray-core/pull/5488#issuecomment-3711430369

For https://github.com/XTLS/Xray-core/issues/4422

common/platform/filesystem/asset_tools.go

@@ -0,0 +1,52 @@
+package filesystem
+
+func DecodeVarint(buf []byte) (x uint64, n int) {
+	for shift := uint(0); shift < 64; shift += 7 {
+		if n >= len(buf) {
+			return 0, 0
+		}
+		b := uint64(buf[n])
+		n++
+		x |= (b & 0x7F) << shift
+		if (b & 0x80) == 0 {
+			return x, n
+		}
+	}
+
+	// The number is too large to represent in a 64-bit value.
+	return 0, 0
+}
+
+func Find(data, code []byte) []byte {
+	codeL := len(code)
+	if codeL == 0 {
+		return nil
+	}
+	for {
+		dataL := len(data)
+		if dataL < 2 {
+			return nil
+		}
+		x, y := DecodeVarint(data[1:])
+		if x == 0 && y == 0 {
+			return nil
+		}
+		headL, bodyL := 1+y, int(x)
+		dataL -= headL
+		if dataL < bodyL {
+			return nil
+		}
+		data = data[headL:]
+		if int(data[1]) == codeL {
+			for i := 0; i < codeL && data[2+i] == code[i]; i++ {
+				if i+1 == codeL {
+					return data[:bodyL]
+				}
+			}
+		}
+		if dataL == bodyL {
+			return nil
+		}
+		data = data[bodyL:]
+	}
+}