SLICC: Remove external_type for structures

SLICC: Remove external_type for structures
In SLICC, in order to define a type a data type for which it should not
generate any code, the keyword external_type is used. For those data types for
which code should be generated, the keyword structure is used. This patch
eliminates the use of keyword external_type for defining structures. structure
key word can now have an optional attribute external, which would be used for
figuring out whether or not to generate the code for this structure. Also, now
structures can have functions as well data members in them.