Support for type-checking

[Parker]

I recently came across this post by Tom Schrijvers which describes an extension to prolog that allows static type-checking.
http://tomschrijvers.blogspot.com/2008/02/types-for-prolog.htm

The features I particularly like are:
- It is optional. The user isn't forced to use types.
- After an initial type check there is no further runtime overhead.

This would be a great feature for Logtalk.

Discussed in forum here: http://forums.logtalk.org/viewtopic.php?f=14&t=30