WL#2901: Add a bit to String class to flag a constant

The task is to add a bit to String class to flag that this is 
a constant that will not change during the evaluation of the 
statement. This allows us to avoid memory allocation by replacing 
some String->copy() with a ->set() based on the argument. Same for 
LEX_STRING, as we do now a extra allocations in 'item_name'.

Rationale
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Code cleanup task requested by Monty.