Speed up token filtering

I told that we are not going to see more easy speed-ups but turns out I was wrong :)

The token filtering loop in ```popFront``` was a performance sucking pump as it did the work that is already done inside lexXYZ. Thus I short-circuited the relevant paths insde of ```advance``` based on flags and gained another 4-5% of speed over the pull #32.

P.S. I constantly mess your indention. I'm using 4 spaces as does the phobos and druntime. I've seen some tabs in the source but dunno if that's a fault on my or your side of things.

dmitry@dmitry-VirtualBox ~/Dscanner $ avgtime -q -r 400 ./dscanner-ldc --tokenCount phobos/std/datetime.d
------------------------
Total time (ms): 9262.23
Repetitions    : 400
Sample mode    : 23 (181 ocurrences)
Median time    : 23.073
Avg time       : 23.1556
Std dev.       : 0.96712
Minimum        : 21.571
Maximum        : 32.811
95% conf.int.  : [21.2601, 25.0511]  e = 1.89552
99% conf.int.  : [20.6644, 25.6467]  e = 2.49114
EstimatedAvg95%: [23.0608, 23.2503]  e = 0.0947761
EstimatedAvg99%: [23.031, 23.2801]  e = 0.124557


dmitry@dmitry-VirtualBox ~/Dscanner $ avgtime -q -r 400 ./pull32-dscanner --tokenCount phobos/std/datetime.d
------------------------
Total time (ms): 9670.87
Repetitions    : 400
Sample mode    : 23 (181 ocurrences)
Median time    : 24.0055
Avg time       : 24.1772
Std dev.       : 1.24027
Minimum        : 22.407
Maximum        : 36.968
95% conf.int.  : [21.7463, 26.6081]  e = 2.43088
99% conf.int.  : [20.9825, 27.3719]  e = 3.19472
EstimatedAvg95%: [24.0556, 24.2987]  e = 0.121544
EstimatedAvg99%: [24.0174, 24.3369]  e = 0.159736