An authentic structure type is one whose instances cannot be
impersonated or chaperoned. The intended use of `prop:authentic` is to
annotate a library-private data structure where impersonators are
never needed internally for the data structure, and the declaration
lets the compiler produce less code and fewer branches by omitting
impersonator support.
A `#:name` identifier picks the name that is bound to static
information about a structure type. An `#:extra-name` identifier
specifies an additional name to be bound to the information.
This pair of options is analogous to `#:constructor-name`
and `#:extra-constructor-name`.
Based on Jen Axel's suggestion and implementation.
Closes#1309
Port `examples`, `interactions`, etc., to use the new `examples`
form of `scribble/examples`. The main intended effect is to ensure
that errors are produced by examples only as specifically
indicated.
