Implement ShExPath

[labra]

Implement ShExPath to improve error messages.

Using ShExPath, it will be easier to improve readability of error messages.

For example, is we have a shape

<Person> {
 :worksFor { 
   :name xsd:string 
 }
}

and RDF:

:alice :worksFor :c .
:c :name 23 .

and try to validate :alice@:User we can show the message:

:alice@!:User, 
Reason: 
23 doesn't conform to /@<User>/:worksFor/@<Company>/:name