Improve performance

[live627]

I have identified three main bottlenecks when profiling the code with xdebug:

1. array_shift(): The array of tokens ended up being renumbered on each iteration. So we instead set consumed tokens to null.
2. mb_substr(): Using mutltibyte here is unnecessary and overkill, when we can just operate directly on the string.
3. Liquid::get(): This one surprised me, but due to the number of calls, here it is. Shave off more processing time by accessing the config array directly.

[fhferreira]

+1

[sanmai]

Can you merge with the master branch please?

I would also appreciate if you could avoid force pushes: it prevents me from reviewing changes added since last review, and I have to effectively begin from scratch. Not great.

[sanmai]

I think I have to return to Liquid::$config again.