Ported lax_parse() to C, 15% faster

Ported lax_parse() to C and cleaned up prior code. New benchmark results:

```
                 user     system      total        real
Liquid:      0.000000   0.000000  26.770000 ( 26.820869)
Liquid-C:    0.000000   0.000000  22.710000 ( 22.767058)
Ratio: 84.88561094588167%
```

Passes all current Liquid tests. Thoughts?

@camilo 
@dylanahsmith 

For ultimate roflscale (see Knuth's first book on linear search) you could set `end` to a quoted fragment (and set it back after this loop). Then when you hit a quoted fragment, you check whether you're at the end. Eliminating `cur < end`. Lol I don't think I actually want you to do this.

but #scale