Use string interning

[simonthum]

trafficstars

This cuts about 20% of parsing cost and probably more in final object graph size, depending on input data.

In terms of the LoadLarge benchmark:

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
LoadLarge before	136.9 ms	3.11 ms	9.03 ms	7500.0000	3500.0000	1500.0000	39.65 MB
LoadLarge after	114.4 ms	3.70 ms	10.67 ms	6000.0000	2000.0000	1000.0000	39.65 MB

This is just a simple string.Intern call. It might make sense to emplyor pass a pool, like a XmlNameTable does with XML, to save on scalar objects not just strings.