Resource counting perf

In Shopify, profiles are showing that resource_limits_reached?/increment_used_resources are taking a disproportionately large amount of time (larger than the synthetic profiles here). To optimize them, this PR uses a class with ivars for each limit rather than a hash.

old synthetic profile:

```
==================================
  Mode: cpu(1000)
  Samples: 2459 (0.00% miss rate)
  GC: 562 (22.85%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
...
        30   (1.2%)          30   (1.2%)     Liquid::Context#resource_limits_reached?
...
        24   (1.0%)          24   (1.0%)     Liquid::Context#increment_used_resources
...
```

new:
```
==================================
  Mode: cpu(1000)
  Samples: 2632 (0.00% miss rate)
  GC: 561 (21.31%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
...
        24   (0.9%)          24   (0.9%)     Liquid::ResourceLimits#increment_for
...
        15   (0.6%)          15   (0.6%)     Liquid::ResourceLimits#reached?
...
```

So, `increment_used_resources` is a little faster (because of instance_of vs kind_of), and `resource_limits_reached?` is much faster.
render should be returning strings.  If type checking is slow, then we could coerce the render output to a string and just call `.length` on it rather than checking for other types.