New SchemaWalker, KeywordWalker

The general idea seems to be there.

Based on the code of SyntaxProcessor and SyntaxChecker. Both of these already
have all the logic to correctly walk schemas, and only schemas. Now all that is
needed is a more generic implementation.

Spawned from the discussion in issue #41.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>