Implement general base conversion. Given a number in base **a**,
represented as a sequence of digits, convert it to base **b**.

## Note
- Try to implement the conversion yourself.
  Do not use something else to perform the conversion for you.

## About [Positional Notation](https://en.wikipedia.org/wiki/Positional_notation)

In positional notation, a number in base **b** can be understood as a linear
combination of powers of **b**.

The number 42, *in base 10*, means:

(4 * 10^1) + (2 * 10^0)

The number 101010, *in base 2*, means:

(1 * 2^5) + (0 * 2^4) + (1 * 2^3) + (0 * 2^2) + (1 * 2^1) + (0 * 2^0)

The number 1120, *in base 3*, means:

(1 * 3^3) + (1 * 3^2) + (2 * 3^1) + (0 * 3^0)

I think you got the idea!


*Yes. Those three numbers above are exactly the same. Congratulations!*