Implement astype(dtype: DType) for simple type casting

* See https://numpy.org/doc/stable/reference/generated/numpy.ndarray.astype.html

Example:

```ts
import { array } from 'vectorious';

const x = array([1, 2, 2.5]);

x.astype('int8')
/*
array([ 1, 2, 2 ], dtype=int8)
*/
```

No source files or tests are named. Start by locating the array implementation and existing dtype handling, then compare the requested behavior with NumPy's ndarray.astype documentation. Done means array([1, 2, 2.5]).astype('int8') produces values [1, 2, 2] with dtype int8.

Written by the indexing model from the issue text.