Speed up Tokenizer by using StringScanner.

Slicing the string after each token made the tokenizing process
really slow.

I eliminated this inneficency by using Ruby's builtin StringScanner,
that's well suited for this task.

Before this change, it took about 14 seconds to tokenize a 3000-line
JavaScript file.  Now it takes just about 0.4 seconds.  That's about
30 x speedup.