[Ur] Proposed location literals to track caller's location in error msgs.

[Gabriel Riba]

The simplest way to add the caller's location is to have a location 
literal, in the spirit of PHP's __LINE__, and pass it as a string parameter.

I am going to use the mlyacc error span meccanism and name the literal 
_LOC_ (LOC surrounded by a single underscore).

Addtion to "urweb.lex":

<INITIAL> "_LOC_" => (Tokens.LOCATION (pos yypos, pos yypos + size yytext));


Addition to "urweb.grm":

* In the terminals %term clause add

  | LOCATION

* In the "eterm" production clause add

        | LOCATION    (let
                         val strLoc = ErrorMsg.spanToString (s 
(LOCATIONleft, LOCATIONright))
                      in
                         (EPrim (Prim.String (Prim.Normal, strLoc)), s 
(LOCATIONleft, LOCATIONright))
                      end)










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