Use C strict parser externally

The strict parser was already written – this exposes it to Ruby and uses it to speed up `Expression.parse`, falling back to the original in the case of bad syntax.

Improvement:

```
ruby -----------------------------------
              parse:       37.9 (±18.5%) i/s -       1083 in  30.035048s
        parse & run:       20.4 (±14.7%) i/s -        582 in  30.056232s

master ---------------------------------
              parse:       68.0 (±19.1%) i/s -       1938 in  30.055672s
        parse & run:       27.8 (±18.0%) i/s -        798 in  30.038484s

expression-parsing ---------------------
              parse:       82.1 (±19.5%) i/s -       2331 in  29.944101s
        parse & run:       32.9 (±15.2%) i/s -        950 in  30.044010s
```

@Shopify/liquid