Dynamically sized bytes type.

### What is wrong?

Dynamically sized byte arrays are commonly used and can be found in the public interfaces of many contracts. For example, it's used [here](https://github.com/Uniswap/uniswap-v2-core/blob/4dd59067c76dea4a0e8e4bfdda41877a6b16dedc/contracts/UniswapV2Pair.sol#L159) in the Uniswap core contracts. Since it's so common, we need to deal with it somehow.

See #258 for discussion about the future of bytes in Fe. For now tho, we may just want to add a type similar to `stringN` for practical reasons. We can always remove it at a future date.

### How can it be fixed

The easiest way to support bytes would be to do what we currently do with strings. That is, set a limit on the size of the string, but keep it dynamic otherwise.

Start by reading the discussion in #258 and examining the existing stringN support referenced in the issue. Define a bounded, dynamically sized bytes type for public contract interfaces, and verify that the resulting type behaves consistently with the current string approach.

Written by the indexing model from the issue text.