Function to determine the number of bytes in a datashape.

It would be nice to know how large our data is without looking at the data itself

```
>>> nbytes('2 * 2 * int64')
32
```

Generally assume the minimum storage required

```
>>> nbytes('{x: int32, y: int32}')
8
```

Accept that this is a hard task to do in general

```
>>> nbytes('string')
TypeError(...)
```
