PR: Add can-cast, broadcast-arrays and broadcast-to

[steff456]

This PR

• Adds specification for can_cast, which determines if a type conversion is allowed.
• Adds specification for broadcast_arrays, which broadcast any number of arrays against each other.
• Adds specification for broadcast_to, which broadcast an array to a new shape.

Notes

• For can_cast the minimal set of arguments was defined, omitting the casting argument which is currently supported in NumPy, CuPy and JAX, but which is not supported according to the type promotion specification. (https://github.com/data-apis/array-api-comparison/blob/main/signatures/data-types/can_cast.md)
• For can_cast the type of the first argument was reduced to the union of dtype and array, removing scalar support as supported by NumPy, Cupy and JAX. Scalars do not participate in type promotion according to the current array API specification. This is in contrast to NumPy, which uses value-based promotion.
• For broadcast_to the minimal set of arguments was defined, omitting the subok keyword argument currently supported
  in NumPy, chunks keyword supported by Dask, and out keyword supported by MXNet. (https://github.com/data-apis/array-api-comparison/blob/main/signatures/data-types/broadcast_to.md)
• For broadcast_arrays, the minimal set of arguments was defined, omitting the argument subok currently supported in NumPy. (https://github.com/data-apis/array-api-comparison/blob/main/signatures/data-types/broadcast_arrays.md)

[rgommers]

Thanks @steff456. Could you add a Notes section to the PR description describing any differences between libraries? As well as links to signature comparisons (e.g. https://github.com/data-apis/array-api-comparison/blob/59a48db415a1eae754e7feaa8d340b1d6f2b2786/signatures/data-types/can_cast.md)?

[rgommers]

This needs some changes; the most important points need comparison data before we can decide.

[asmeurer]

I noted some issues here that came up from my script that parses the function signatures. There are also similar issues in iinfo and finfo, if you want to fix them here (otherwise I will open a new PR for them). Some comments were marked as resolved but weren't actually resolved.

[steff456]

I can update that in this PR. I haven't make a push with the changes but I already made them locally (:

[asmeurer]

I have #133 but feel free to make the changes here too if you already made them.

[kgryte]

Thanks for working on this @steff456! Mainly formatting and phrasing nits. The main question for me is whether can_cast should support dtypes and arrays for both from and to.

[rgommers]

LGTM too, merging. Thanks @steff456. And thanks for the review @kgryte