use StringScanner library for parsing

[gemmaro]

before:

$ ruby bench.rb 
Warming up --------------------------------------
               parse     1.000  i/100ms
Calculating -------------------------------------
               parse      1.549  (± 0.0%) i/s -      8.000  in   5.164986s

after:

$ ruby bench.rb 
Warming up --------------------------------------
               parse     1.000  i/100ms
Calculating -------------------------------------
               parse      5.354  (± 0.0%) i/s -     27.000  in   5.043558s