Speed up symbol lookup by using hash code as major key.

This pull request speeds up symbol lookup.  It removes about 6% off the time to run ```julia runtests.jl linalg1``` for me.  Here are times I measured:.  

          |        |       |       |       |       | average  | stdev
----------|--------|-------|-------|-------|-------|----------|------
original  | 50.2   | 49.5  | 50.29 | 49.76 | 49.61 | 49.87    | 0.35
PR        | 46.83  | 46.63 | 47.21 | 47.03 | 46.51 | 46.84    | 0.29

The change uses the hash code as the major sort key, which enables more comparisons to be done in a single word comparison instead of using ```strcmp```.   

The PR also removes the need for an ifdef by replacing a constant with a formula that has the same value.