Scribble: add 'grouper and 'hidden-number part style properties

Also, clean up documentation and implementation of 'unnumbered and
'hidden.
Scribble: add `scribble/book' and `scribble/report' languages

These languages set the default class for Latex rendering, and also
adjust Latex commands so that top-level sections are chapters, etc.
fix prototype when the C compiler has no "no inline" annotation