步行
已弃用，支持
示例用法：
var walk = require ( 'vz.walk' ) ,
yieldify = require ( 'vz.yieldify' ) ,
fs = require ( 'fs' ) ,
exists = yieldify ( fs . exists ) ;
walk ( function * ( ) {
if ( yield exists ( 'foo.bar' ) ) console . log ( 'foo.bar exists!' ) ;
else console . log ( 'foo.bar doesn\'t exist' ) ;
} ) ;
