Added test to exercise union type transformation for the 'len' operator
Added test to exercise union type transformation for the 'card' operator
Replaced the size operator in the IR with a 'len' and 'card' operator
Added some test to exercise generation of the sub sequence operator when the sequence is a string
Added Java codegen library implementation of the sub sequence operator when the sequence is a string
Made codegen Java library implementation of the IO library compatible with the generated Java code
Added union type transformation for the 'len' and 'card' operators
Added test to exercise generation of the call to VDMUtil`val2seq_of_char
Added Java codegen runtime implementation of VDMUtil`val2seq_of_char