`Const` from shape-castable

Summary

Allow passing a shape-castable to Const.

We currently have two incompatible syntaxes for making a constant, depending on whether it's made from a Shape or a shape-castable. The former uses Const(value, shape), while the latter requires shape.const(value).

Making Const accept shape-castables means we'll have a syntax that works for all shape-likes, reducing the need to special case for shape-castables.

Guide- and reference-level explanation

Const(value, shape) checks whether shape is a shape-castable and returns shape.const(value) when this is the case.

Drawbacks

A Const() constructor sometimes returning non-Const objects can be confusing.
- Signal() already behaves this way.

Rationale and alternatives

This is consistent with how Signal() handles shape-castables.

Alternatives:

Do not do this. Require code that makes constants from a passed shape-like to check whether it got passed a shape-castable or not and pick the appropriate syntax.