Minimize the number of HashMap allocations/insertions in the Infoset

Performance testing found that the slot removal made in DAFFODIL-1854 caused performance degradation, sometimes up to 20% decreases. Profiling found that some of this is due to the allocations of the HashMap used for quickly looking up elements in the Infoset by QName. We should only allocate hashmaps when it's possible an element could be be used in an expression, which should improve performance.