Major Description
In order to support the new types functionality described in http://wiki.apache.org/pig/PigTypesFunctionalSpec, the parse needs to change in the following ways:
1) AS needs to support types in addition to aliases. So where previously it was legal to say:
a = load 'myfile' as a, b, c;
it will now also be legal to say
a = load 'myfile' as a integer, b float, c chararray;
2) Non string constants need to be supported. This includes non-string atomic types (integer, long, float, double) and the non-atomic types bags, tuples, and maps.
3) A cast operator needs to be added so that fields can be explicitly casted.
4) Changes to DEFINE, to allow users to declare arguments and return types for UDFs