embed: remove type spelling restrictions

[rsc]

In #43217 (comment) I suggested two changes:

• Drop the import restriction for embed.
• Drop the type spelling restrictions for embed.

The reaction is for the most part overwhelmingly positive. But dropping the import restriction has generated some important discussion, while dropping the type spelling restriction seems completely uncontroversial.

I've forked this issue off of #43217 to track only dropping the type spelling restriction. Then #43217 can be only about dropping the import restriction.

[rsc]

(The Likely Accept is inherited from #43217, which this was split off from.)

[gopherbot]

Change https://golang.org/cl/282715 mentions this issue: cmd/compile: allow embed into any string or byte slice type

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://golang.org/cl/288072 mentions this issue: embed: update docs for proposal tweaks