ffi/unsafe: add `_stdbool`
add `racket/undefined`

The `(letrec ([x x]) x)` pattern for getting the undefined value will
cease to work in a near-future version of Racket (ater
v6.0.1). Instead, the initial refernce to `x` will produce a "variable
used before its definition" error.

For libraries that need an "undefined" value, this new library
provides `undefined`, and the library will continue to work in future
Racket versions.

The library also provides a `check-no-undefined` operation that
normally should be wrapped around an expression to keep it from
producing `undefined`. For example, the class system initializes
object fields with `undefined`, but it could (and will, eventually)
wrap each access to a field within `class` to check that the field's
value is not `undefined`.