Require a `crate` field for every asset that is a crate

[benfrankel]

This will help determine an accurate "latest supported Bevy version" for every crate. Related issue: #297.

Some assets have multiple crates, in which case any one of them can be used.

[benfrankel]

Current status:

• Some crates in Bevy Assets have not been released on crates.io, so they still have no crate = "..." field. Every other crate now has a crate field.
• crate is an optional field for now to accommodate those unreleased crates. Making it a required field is blocked on Question: How should these crates be handled? #457.
• New crates that are added to Bevy Assets ought to include a crate field.