Need a way to define passes that do not auto-generate transformers.

[akeep]

In the current version of the nanopass framework, passes with always auto-generate missing transformers and assume that that is what was intended by the programmer, even if the programmer simply missed a case, or misspecified a cata-morphism.

It should be possible to disable this when the programmer wishes to see instead receive an error for a missing transformer.

(requested, most recently, by Tim Zakian--but also by Kent Dybvig, Eric Holk, etc.)