Add scanner benchmark.

Current values on my machine:

Running benchmarks for 2 seconds on 1388 bytes (bench.dtl)...
Running erlydtl_new_scanner, hold on... ok
Running erlydtl_scanner, hold on... ok
erlydtl_new_scanner scanned   4171 times in 2.000 seconds @ 2826.8 Kbytes/sec
  min 0.000 max 0.002 avg 0.000480
erlydtl_scanner scanned   4886 times in 2.000 seconds @ 3311.4 Kbytes/sec
  min 0.000 max 0.003 avg 0.000409
Deltas
  Count     :     -715.000  -14.6%
  Total time:        0.000    0.0%
  Min       :        0.000      ~%
  Max       :       -0.001  -33.3%
  Avg       :        0.000   17.1%
  Kbytes/s  :     -484.580  -14.6%
Done.