Support using `insertSelect` with an entire `Entity`

Sometimes I want to insert entire entities into a table, from a `SELECT` query, including the primary key.[^1] As far as I can tell, this isn't possible with the current `insertSelect` & `Insertion` API - the `PersistEntity` constraint and the types of `(<#)` and `(<&>)` get in the way.

The easiest way to support this is probably with an extra `insertSelectEntity`, to parallel `insertEntityMany` from Persistent. I haven't yet tried to come up with a safe type for such a function.

[^1]: My use case right now actually involves a [`REPLACE`](https://dev.mysql.com/doc/refman/8.0/en/replace.html), as in #86, but it's otherwise the same idea.

Start by reading the insertSelect and Insertion APIs, then compare them with Persistent's insertEntityMany. Determine how the PersistEntity constraint and the types of (<#) and (<&>) prevent inserting an entire Entity, including its primary key, from a SELECT query. Done means a safe API, possibly insertSelectEntity, supports that use case.

Written by the indexing model from the issue text.