Avoid array lookups where simple constants are fine

A 15 minute fix that gets a big performance (about 20-30%)!

The noise is awfully high though, so I'm looking at sample mode mostly it could be bigger or a smaller. What I'm seeing I think is that system call overhead is far higher then the time to lex the thing.

Logs of runs on core i5-3170 at my workplace.
dscanner-meta truly  utilizes meta-programming unlike the current version (dscanner-old):

dmitry@dmitry-pc ~/Dscanner $ avgtime -r 450 -q  ./dscanner-old --tokenCount ../phobos/std/datetime.d 

------------------------
Total time (ms): 8719.33
Repetitions    : 450
Sample mode    : 22 (81 ocurrences)
Median time    : 20.492
Avg time       : 19.3763
Std dev.       : 2.97428
Minimum        : 13.957
Maximum        : 25.352
95% conf.int.  : [13.5468, 25.2058]  e = 5.82948
99% conf.int.  : [11.715, 27.0375]  e = 7.66123
EstimatedAvg95%: [19.1015, 19.6511]  e = 0.274804
EstimatedAvg99%: [19.0151, 19.7374]  e = 0.361154


dmitry@dmitry-pc ~/Dscanner $ avgtime -r 450 -q  ./dscanner-meta --tokenCount ../phobos/std/datetime.d 

------------------------
Total time (ms): 8114.66
Repetitions    : 450
Sample mode    : 15 (117 ocurrences)
Median time    : 17.304
Avg time       : 18.0326
Std dev.       : 2.89978
Minimum        : 12.55
Maximum        : 23.87
95% conf.int.  : [12.3491, 23.716]  e = 5.68346
99% conf.int.  : [10.5633, 25.5019]  e = 7.46933
EstimatedAvg95%: [17.7647, 18.3005]  e = 0.267921
EstimatedAvg99%: [17.6805, 18.3847]  e = 0.352107
dmitry@dmitry-pc ~/Dscanner $