Move linear algebra APIs to an extension

[kgryte]

This PR

• builds on gh-165 whereby only a small subset of linear algebra APIs are present in the top-level namespace and the rest are specified in a dedicated linalg sub-namespace as an extension to the array API standard.

Notes

• How extensions should be authored and consumed remains to be decided. This PR should lay the groundwork for further specification.

[asmeurer]

Did we decide to mirror the functions in the top-level namespace?

[rgommers]

Did we decide to mirror the functions in the top-level namespace?

I believe that was indeed the preferred option. Discussion at #149 (review) followed by a discussion in the May 6th call where people seemed happy with @kgryte's proposal as implemented in this PR.

For others, it may help to capture how the PR in its current state is rendered. The API specification for the main namespace has these functions:

Extensions is a separate section:

And that section has all linear algebra functions in it (including the 5 in the main namespace, which are aliased).

[rgommers]

LGTM, thanks @kgryte

[rgommers]

Let me merge this, since people are busy implementing this in at least PyTorch. If there are more comments, let's deal with those in a follow-up PR.