keyword object support

[nyuichi]

Currently there are two de-facto standards of keyword object in the scheme world: prefixed and suffixed. Though one of the scheme technical reports srfi-88 defines suffixed version keyword object, the prefixed keyword object has traditionally been preferred by the lisp and scheme community and in fact widely spread.
IMHO keyword object basically matches neatly with read-table based reader system. From the point of implementer's view, it is more natual to have a prefix to distinguish normal symbols and keywords.