raco pkg: support single-collection packages

A package directory is in single-collection mode when it has an
"info.rkt" file containing a `single-collection' definition.
The string value of `single-collection' provides the collection's
name.
Add 'no-sidebar style property as a style for the main part of a document

The new style property causes the HTML output to not include an ``on this
page'' margin box.
doc repair

Fix "style" where it should be "style property".
ffi/unsafe: doc repair

from Клочков Роман